0001564590-22-007622 | 10-K | XBRL Viewer

The Company	12 Months Ended
Dec. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company Merus N.V. is a clinical-stage oncology company developing innovative antibody therapeutics, headquartered in Utrecht, the Netherlands. Merus US, Inc. is a wholly-owned subsidiary of Merus N.V. located at 139 Main Street, Cambridge, Massachusetts, United States (collectively, the “Company”). Since inception, the Company has generated an accumulated deficit of $466.9 million as of December 31, 2021. The Company expects to continue to incur significant expenses and operating losses for the foreseeable future as its antibody candidates advance through discovery, pre-clinical development and clinical trials and as it seeks regulatory approval and pursues commercialization of any approved antibody candidate. As a result, the Company may need additional financing to support its continuing operations. Until the Company can generate significant revenue from product sales, if ever, the Company expects to finance its operations through public equity offerings, debt financings, or other sources, which may include collaborations, business development and licensing opportunities with third parties. Adequate additional financing may not be available to the Company on acceptable terms, or at all. The Company’s inability to raise capital as and when needed would have a negative impact on its financial condition and ability to pursue its business strategy. The Company will need to generate significant revenues to achieve profitability and may never do so. Based on the Company’s current operating plan, the Company expects that its existing cash, cash equivalents and marketable securities of $430.7 million as of December 31, 2021, will fund the Company’s operations beyond 2024.

The Company

12 Months Ended

Dec. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

1. The Company

Merus N.V. is a clinical-stage oncology company developing innovative antibody therapeutics, headquartered in Utrecht, the Netherlands. Merus US, Inc. is a wholly-owned subsidiary of Merus N.V. located at 139 Main Street, Cambridge, Massachusetts, United States (collectively, the “Company”).

Since inception, the Company has generated an accumulated deficit of $466.9 million as of December 31, 2021. The Company expects to continue to incur significant expenses and operating losses for the foreseeable future as its antibody candidates advance through discovery, pre-clinical development and clinical trials and as it seeks regulatory approval and pursues commercialization of any approved antibody candidate.

As a result, the Company may need additional financing to support its continuing operations. Until the Company can generate significant revenue from product sales, if ever, the Company expects to finance its operations through public equity offerings, debt financings, or other sources, which may include collaborations, business development and licensing opportunities with third parties. Adequate additional financing may not be available to the Company on acceptable terms, or at all. The Company’s inability to raise capital as and when needed would have a negative impact on its financial condition and ability to pursue its business strategy. The Company will need to generate significant revenues to achieve profitability and may never do so.

Based on the Company’s current operating plan, the Company expects that its existing cash, cash equivalents and marketable securities of $430.7 million as of December 31, 2021, will fund the Company’s operations beyond 2024.

Employee Benefit Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning balance	3,748,911
Number of Options, Granted	1,522,210
Number of Options, Exercised	(1,104,029)
Number of Options, Forfeited or expired	(182,625)
Number of Options, Outstanding, Ending balance	3,984,467	3,748,911
Number of Options, Exercisable	1,841,578
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 15.59
Weighted Average Exercise Price per Share, Granted	23.78
Weighted Average Exercise Price per Share, Exercised	15.90
Weighted Average Exercise Price per Share, Forfeited or expired	20.43
Weighted Average Exercise Price per Share, Outstanding, Ending balance	18.40	$ 15.59
Weighted Average Exercise Price per Share, Exercisable	$ 15.64
Weighted Average Remaining Contractual Term, Outstanding	7 years 4 months 24 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 9 months 18 days
Aggregate Intrinsic Value, Outstanding	$ 53,445
Aggregate Intrinsic Value, Exercisable	$ 29,825
Weighted-average fair value of options granted	$ 17.01	$ 11.75

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Lilly	Common Stock [Member]	Common Stock [Member] Lilly	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Lilly	Accumulated Deficit [Member]	Accumulated Deficit [Member] Lilly	Accumulated Other Comprehensive (Loss) Income [Member]	Accumulated Other Comprehensive (Loss) Income [Member] Lilly
Beginning balance at Dec. 31, 2018	$ 105,857		$ 2,366		$ 360,045		$ (259,448)		$ 2,894
Beginning balance, shares at Dec. 31, 2018			23,358,977
Issuance of common stock, net	74,020		$ 546		73,474		0		0
Issuance of common stock,net, shares			5,462,500
Exercise of stock options and vesting of restricted stock units	48		$ 6		42		0		0
Exercise of stock options and vesting of restricted stock units, shares			60,740
Short-swing profit disgorgement	0
Stock-based compensation	7,834		$ 0		7,834		0		0
Currency translation adjustment	(1,308)		0		0		0		(1,308)
Net loss	(55,151)		0		0		(55,151)		0
Ending balance at Dec. 31, 2019	131,300		$ 2,918		441,395		(314,599)		1,586
Ending balance, shares at Dec. 31, 2019			28,882,217
Issuance of common stock, net	38,171		$ 265		37,906		0		0
Issuance of common stock,net, shares			2,451,281
Exercise of stock options and vesting of restricted stock units	1,448		$ 28		1,420		0		0
Exercise of stock options and vesting of restricted stock units, shares			269,455
Short-swing profit disgorgement	0
Stock-based compensation	9,372		$ 0		9,372		0		0
Currency translation adjustment	7,485		0		0		0		7,485
Net loss	(85,513)		0		0		(85,513)		0
Ending balance at Dec. 31, 2020	102,263		$ 3,211		490,093		(400,112)		9,071
Ending balance, shares at Dec. 31, 2020			31,602,953
Issuance of common stock, net	248,058	$ 16,477	$ 1,072	$ 77	246,986	$ 16,400	0	$ 0	0	$ 0
Issuance of common stock,net, shares			10,014,354	706,834
Exercise of stock options and vesting of restricted stock units	17,423		$ 121		17,302		0		0
Exercise of stock options and vesting of restricted stock units, shares			1,142,911
Repurchase of restricted stock units	(285)		$ 0		(285)		0		0
Short-swing profit disgorgement	282		0		282		0		0
Stock-based compensation	17,091		0		17,091		0		0
Currency translation adjustment	(18,292)		0		0		0		(18,292)
Net loss	(66,816)		0		0		(66,816)		0
Ending balance at Dec. 31, 2021	$ 316,201		$ 4,481		$ 787,869		$ (466,928)		$ (9,221)
Ending balance, shares at Dec. 31, 2021			43,467,052

Prepaid Expenses and Other Assets (Tables)

12 Months Ended

Dec. 31, 2021

Prepaid Expense And Other Assets Current [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following:

	December 31,
	2021		2020
	(In thousands)
Prepaid clinical and manufacturing costs	$	2,146	$	4,971
Prepaid general and administrative costs		2,760		2,460
Interest receivable		367		80
Other		2,175		1,058
Total	$	7,448	$	8,569

Income Taxes - Summary of Components of Loss from Operations Before Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
United States	$ (8,502)	$ (3,832)	$ (1,363)
Netherlands	(58,075)	(81,178)	(53,594)
Loss before income tax expense	$ (66,577)	$ (85,010)	$ (54,957)

- Definition

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

+ References

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+ Details

Employee Benefit Plans - Intrinsic Value of Stock Options Exercised and Vested RSUs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of options exercised	$ 10,774	$ 1,750
RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total fair value of RSUs vested	$ 903	$ 961

Accrued Expenses and Other Liabilities (Tables)

12 Months Ended

Dec. 31, 2021

Payables And Accruals [Abstract]

Summary of Accrued Expenses

Accrued expenses consisted of the following:

	December 31,
	2021		2020
	(In thousands)
Accrued research and development expenses	$	15,174	$	15,372
Accrued personnel costs		4,861		4,854
Accrued general and administrative expenses		1,362		1,566
Other		1,109		11
Accrued expenses	$	22,506	$	21,803

Investments in Debt Securities - Summary of Debt Securities by Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 194,971	$ 62,327
AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	5,684
AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	63,146
A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	126,141
Money market funds [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	5,684	10,156
Money market funds [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	5,684
Money market funds [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
Money market funds [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
Corporate paper and notes [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	155,039	27,978
Corporate paper and notes [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
Corporate paper and notes [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	28,898
Corporate paper and notes [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	126,141
U.S. government agency securities [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	2,093	9,150
U.S. government agency securities [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
U.S. government agency securities [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	2,093
U.S. government agency securities [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
U.S. treasuries [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	32,155	$ 15,043
U.S. treasuries [member] \| AAA [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	0
U.S. treasuries [member] \| AA- to AA+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	32,155
U.S. treasuries [member] \| A- to A+ [member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 0

Employee Benefit Plans - Summary of Key Weighted Average Assumptions used in Pricing Model on date of Grant for Options Granted to Employees (Details) - Stock Option Activity	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	0.60%	1.30%	2.50%
Contractual life of options (years)	10 years	10 years	10 years
Expected term of options (years)	6 years 1 month 6 days	6 years 3 months 18 days
Expected volatility of underlying stock	85.70%	86.10%	87.00%
Expected dividend yield	0.00%	0.00%	0.00%

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

Summary of Estimated Useful Lives of Property and Equipment

The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:

Asset Category		Useful Lives
Laboratory equipment		5 years
Office furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or term of lease

Investments in Debt Securities - Summary of Classification of Debt Securities in Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	$ 194,971	$ 62,327
Cash equivalents [member]
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	5,684	17,654
Current marketable securities [member]
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	168,990	44,673
Non-current marketable securities [member]
Debt Securities Held To Maturity Nonaccrual [Line Items]
Balance	$ 20,297	$ 0

The Company - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (466,928)	$ (400,112)
Cash, cash equivalents and marketable securities	$ 430,700

- Definition

The cumulative amount of the reporting entity's undistributed earnings or deficit.

+ References + Details

Prepaid Expenses and Other Assets

12 Months Ended

Dec. 31, 2021

Prepaid Expense And Other Assets Current [Abstract]

Prepaid Expenses and Other Assets

4. Prepaid Expenses and Other Assets

Prepaid expenses and other current assets consisted of the following:

	December 31,
	2021		2020
	(In thousands)
Prepaid clinical and manufacturing costs	$	2,146	$	4,971
Prepaid general and administrative costs		2,760		2,460
Interest receivable		367		80
Other		2,175		1,058
Total	$	7,448	$	8,569

Restricted cash included in other assets totaled $0.3 million and $0.2 million as of December 31, 2021 and 2020, respectively. The nature of the restriction relates to amounts held as collateral for a credit card borrowing arrangement.

Intangible Assets, Net - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Intangible Assets Net Excluding Goodwill [Abstract]
Amortization of intangible assets	$ 240	$ 279	$ 236

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (66,816)	$ (85,513)	$ (55,151)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization of property and equipment	1,245	1,165	902
Amortization of intangible assets	240	279	236
Foreign exchange losses (gains)	(27,703)	8,957	(1,068)
Stock-based compensation expense	17,091	9,372	7,834
Amortization of discount on investments	450	40	(531)
Deferred tax benefit	(7)	(122)	(89)
Changes in operating assets and liabilities:
Accounts receivable	(4,991)	1,149	633
Operating lease right-of-use assets and lease obligations	(24)	0	(112)
Prepaid expenses and other current assets	(1,092)	(2,895)	(1,029)
Accounts payable	10,715	(110)	22
Accrued expenses and other liabilities	2,127	7,023	3,317
Deferred revenue	9,138	(19,246)	(18,012)
Net cash used in operating activities	(59,627)	(79,901)	(63,048)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities	(215,839)	(66,845)	(60,413)
Proceeds from maturities of marketable securities	70,086	66,646	87,183
Purchases of intangible assets	0	0	(375)
Purchases of property and equipment	(870)	(1,287)	(2,223)
Net cash provided by (used in) investing activities	(146,623)	(1,486)	24,172
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of offering costs	(572)	0	0
Proceeds from issuance of common stock, net	248,630	38,072	74,184
Proceeds from stock options exercised	17,423	1,448	48
Repurchase of restricted stock units	(285)	0	0
Short-swing profit disgorgement	282	0	0
Net cash provided by financing activities	281,955	39,520	74,232
Foreign exchange impact on cash, cash equivalents and restricted cash	2,761	7,337	(2,273)
Net increase (decrease) in cash, cash equivalents and restricted cash	78,466	(34,530)	33,083
Cash, cash equivalents, and restricted cash, beginning of period	163,283	197,813	164,730
Cash, cash equivalents, and restricted cash, end of period	241,749	163,283	197,813
SUPPLEMENTAL DISCLOSURES:
Non-cash right-of-use assets acquired from operating lease obligations	2,626	0	3,875
Income taxes paid	(635)	(322)	(320)
Non-cash purchases of property, equipment and intangibles	0	36	187
Non-cash issuance of stock options	573	0	0
Non-cash financing costs	0	71	164
Income tax refunds received	0	24	0
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and cash equivalents	241,435	163,082	197,612
Restricted cash included in other assets	314	201	201
Cash, cash equivalents, and restricted cash, end of period	241,749	163,283	197,813
Lilly
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock - Lilly	$ 16,477	$ 0	$ 0

Loss per Share (Tables)

12 Months Ended

Dec. 31, 2021

Earnings Per Share [Abstract]

Summary of Basic and Diluted Loss per Share

Basic and diluted loss per share allocable to common stockholders are computed as follows:

	Year Ended December 31,
	2021			2020			2019
	(In thousands except per share data)
Net loss	$	(66,816	)	$	(85,513	)	$	(55,151	)
Weighted average shares outstanding		38,638,434			29,256,203			24,218,083
Basic and diluted loss per share allocable to common stockholders	$	(1.73	)	$	(2.92	)	$	(2.28	)

Operating Leases - Additional Information (Details) € in Millions	1 Months Ended		12 Months Ended
	Jul. 31, 2019 EUR (€) m² RenewalTerm	Mar. 31, 2019	Dec. 31, 2021
Lessee Lease Description [Line Items]
Extended lease term		7 years	1 year 6 months
Operating lease, description		Merus US, Inc. entered into a non-cancellable operating lease agreement for office space in Cambridge, Massachusetts. The lease commenced in the second quarter of 2019 and has a term of seven years, and may be extended for another five years. Given the Company’s current plans, the renewal term has not been included in the estimate of the lease term.	Merus N.V. has non-cancellable operating leases for its corporate headquarters in Utrecht, the Netherlands. During the year ended December 31, 2021, the Company signed a lease amendment (the “Amendment”), which extended the lease term by approximately 1.5 years ending at the end of March 2023. The Amendment did not include additional right-of-use other than the extended lease term.
Operating lease, expiration period			March 2023
Additional renewal term			no
Operating lease, extended term of agreement		5 years
Operating lease, option to extend		true
Operating lease, options to extend description		may be extended for another five years.
Scenario Plan [Member] \| Kadans Science Partner XII BV \| Office Building
Lessee Lease Description [Line Items]
Extended lease term	10 years
Area of property available for operating lease \| m²	5,070
Operating lease, renewal term	5 years
Operating lease, number of renewal term \| RenewalTerm	2
Operating lease, termination term	20 years
Operating lease, estimated rent expense \| €	€ 1.3
Operating lease, rent adjustment description	The rent amount is subject to adjustment based on the consumer price index (the “CPI”) beginning on January 1, 2019 through the completion date and then annually thereafter, subject to certain limitations if the CPI is greater than 3.0%.
Scenario Plan [Member] \| Kadans Science Partner XII BV \| Minimum \| Office Building
Lessee Lease Description [Line Items]
Consumer price Index	3.00%

Property and Equipment, net (Tables)

12 Months Ended

Dec. 31, 2021

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2021			2020
	(In thousands)
Laboratory equipment	$	5,583		$	5,695
Office equipment and furniture		1,222			1,300
Leasehold improvements		111			117
Construction in progress		1,056			496
Property and equipment		7,972			7,608
Less: accumulated depreciation and amortization		(4,423	)		(3,493	)
Property and equipment, net	$	3,549		$	4,115

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

Basis of Preparation

The Company prepared its consolidated financial statements in compliance with generally accepted accounting principles in the U.S. ("U.S. GAAP"). Any reference in these notes to applicable guidance is meant to refer to authoritative U.S. GAAP as found in the Accounting Standards Codification ("ASC") and Accounting Standards Update ("ASU") of the Financial Accounting Standards Board ("FASB").

Principles of Consolidation

Subsidiaries are entities controlled by the Company, consisting of Merus N.V.’s wholly owned subsidiary Merus US, Inc. The Company controls an entity when it is exposed to, or has rights to, variable returns from its involvement with the entity and has the ability to affect those returns through its power over the entity. The financial statements of subsidiaries are included in the consolidated financial statements from the date on which control commences until the date on which control ceases. All significant intercompany balances and transactions have been eliminated in consolidation.

Functional and Presentation Currency

Items recorded in each of the Company’s entities are measured using the currency of the primary economic environment in which the entity operates (the "functional currency"). Merus US, Inc.’s functional currency is the U.S. dollar. The functional currency of Merus N.V. is the euro. After measuring foreign currency denominated transactions into an entity’s functional currency, to the extent that a subsidiary’s functional currency differs from its parent, a subsidiary’s financial position and results of operations are translated into its parent’s functional currency. The Company’s consolidated financial position and results of operations are translated into the U.S. dollar as the Company’s reporting currency.

Use of Estimates

The preparation of these consolidated financial statements in accordance with U.S. GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the consolidated financial statements, and the reported amounts of collaboration revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as

defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer.

Accounts receivable represent amounts due from collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection.

Subsequent Events

The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company evaluated all events and transactions through the date these financial statements were filed with the Securities and Exchange Commission.

Fair Value Measurements

Fair value is defined as an exit price, representing the amount that would be received upon the sale of an asset or payment to transfer a liability in an orderly transaction between market participants. Fair value is a market-based measurement that is determined based on assumptions that market participants would use in pricing an asset or liability. A three-tier fair value hierarchy is used to prioritize the inputs in measuring fair value as follows:

•

Level 1 – Quoted market prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

•

Level 2 – Quoted market prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable, either directly or indirectly. Fair value determined through the use of models or other valuation methodologies.

•

Level 3 – Significant unobservable inputs for assets or liabilities that cannot be corroborated by market data. Fair value is determined by the reporting entity’s own assumptions utilizing the best information available and includes situations where there is little market activity for the asset or liability.

The asset’s or liability’s fair value measurement within the fair value hierarchy is based upon the lowest level of any input that is significant to the fair value measurement.

The Company considers its cash, cash equivalents, accounts receivable, marketable securities due with maturities 12 months or less, and accounts payable financial instruments to reflect their fair value given their short maturity and risk profile of the counterparty.

Going Concern

At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern.

The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its clinical-stage programs and its plans to pursue commercialization of any antibody candidate, if approved, and after considering its existing cash, cash equivalents and marketable securities as of December 31, 2021, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. Additional details of the Company’s cash runway is described in Note 1 The Company.

Cash and Cash Equivalents

The Company considers all highly liquid debt securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Instruments subject to restrictions are not included in cash and cash equivalents.

Restricted Cash

The Company maintains certain cash balances restricted to withdrawal or use. Restricted cash includes cash held as collateral for certain contractual agreements and is recorded in other assets in the consolidated balance sheets.

Marketable Securities

The Company classifies marketable securities that are debt securities with a remaining maturity when purchased of greater than three months as held-to-maturity as the Company has the positive intent and ability to hold such debt securities through maturity.

Debt securities that are classified as held-to-maturity are initially recognized and measured at fair value. Subsequent to initial recognition, they are measured at amortized cost using the effective interest rate method. Interest income from these debt securities is included in interest income. Marketable securities are classified as current if their expected maturity is within one year or less of the balance sheet date and non-current if their maturity is beyond one year of the balance sheet date.

Accounts Receivable

Accounts receivable are amounts due from collaboration partners as a result of research and development services provided or milestones achieved but not yet paid.

Allowance for Credit Losses

The Company evaluates its cash equivalents, accounts receivable and held-to-maturity marketable securities financial assets for expected credit losses. Expected credit losses represent the portion of the amortized cost basis of a financial asset that an entity does not expect to collect. An allowance for expected credit losses is meant to reflect a risk of loss even if remote, irrespective of the expectation of collection from a particular issuer or debt security. The Company has not historically experienced any credit losses on any of its financial assets.

With respect to cash equivalents and accounts receivable, given consideration of their short maturity, lack of historical losses and the current environment, the Company concluded there is generally no expected credit losses for these financial assets. With respect to held-to-maturity marketable securities which are comprised of debt securities, the Company evaluates expected credit losses on a pooled basis based on issuer-type which have similar credit risk characteristics. The allowance for credit losses is immaterial for all periods presented.

Property and Equipment

The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:

Asset Category		Useful Lives
Laboratory equipment		5 years
Office furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or term of lease

The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation or amortization, from the related accounts, and the resulting gain or loss is reflected in the results of operations.

Intangible Assets

Intangible assets are identifiable non-monetary assets without physical substance. An asset is a resource that is controlled by the enterprise as a result of past events (for example, purchase or self-creation) and from which future economic benefits (inflows of cash or other assets) are expected. The useful lives of intangible assets are assessed to be definite-lived and amortized over the useful economic life. The Company’s intangible assets are comprised of purchased licenses to intellectual property and software licenses.

Impairment of Long-Lived Assets

The Company reviews long-lived assets to be held and used, including property and equipment, operating lease right-of-use assets and definite-lived intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets or asset group may not be recoverable.

Evaluation of recoverability is first based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. No such impairments were recorded in 2021, 2020 or 2019.

Leases

The Company determines if an arrangement is or contains a lease at inception. For leases with a term of 12 months or less, the Company does not recognize a right-of-use asset or lease liability. The Company does not have any finance leases.

Right-of-use assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease, and excludes non-lease payments. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As the Company’s leases typically do not provide an implicit rate, the Company uses an estimate of its incremental borrowing rate based on the information available at the lease commencement date in determining the present value of lease payments.

Operating lease right-of-use assets also include the effect of any lease payments made and excludes lease incentives. The lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Operating lease expense is recognized on a straight-line basis over the lease term.

The Company has real estate operating lease agreements with lease and non-lease components, which are generally accounted for separately as operating lease costs and variable lease costs. Non-lease components in real estate leases refer to services provided by the lessor related to the premises. Fixed and variable lease payments are both allocated to lease and non-lease components. The allocation is determined on a relative fair value basis of the services provided relative to the operating lease of premises. With respect to equipment leases, the Company has elected not to allocate payments amongst lease and non-lease components as a practical expedient as afforded under ASC 842, Leases.

Income Taxes

Deferred Taxes

The Company records deferred taxes to recognize the future effects of temporary differences between the tax basis and financial statement carrying amount of assets and liabilities. The Company measures the deferred taxes using enacted tax rates expected to apply when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available, reversing taxable temporary differences and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made.

Unrecognized Tax Benefits

The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the technical merits, facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to an underpayment of income taxes, if applicable, as a component of income tax expense.

Revenue Recognition

The Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for an arrangement, the Company performs the following five step analysis:

identify the contract(s) with a customer;

ii.

identify the performance obligations in the contract;

iii.

determine the transaction price;

iv.

allocate the transaction price to the performance obligations in the contract; and

recognize revenue when (or as) the entity satisfies a performance obligation.

The Company has entered into collaboration and license agreements, which are within the scope of ASC 606, Revenue from Contracts with Customers, to discover, develop, manufacture and commercialize product candidates. The terms of these agreements typically contain multiple promises or obligations, which may include: (i) licenses, or options to obtain licenses, to product candidates or future product candidates directed to specific targets (referred to as “exclusive licenses”) and (ii) research and development activities to be

performed on behalf of the collaboration partner related to the licensed targets. The Company also derives revenue from government grants.

As part of the accounting for these arrangements, the Company must use judgment to determine:

the number of performance obligations based on the determination under step (ii) above and whether those performance obligations are distinct from other performance obligations in the contract;

the transaction price under step (iii) above;

the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above;

whether the combined performance obligation is satisfied over time or at a point in time in step (v) above; and

the appropriate method for measuring progress toward complete satisfaction of a performance obligation in step (v) above.

The Company uses judgment to determine whether milestones or other variable consideration, except for sales-based royalties, should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. In validating its estimated stand-alone selling price, the Company evaluates whether changes in the key assumptions used to determine its estimated stand-alone selling price will have a significant effect on the allocation of arrangement consideration between performance obligations.

Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as unbilled receivables.

Exclusive Licenses

If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license.

In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the rights and obligations set out in the contract, the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promises, whether the value of the license is dependent on the unsatisfied promises, whether there are other vendors that could provide the remaining promises, and whether it is separately identifiable from the remaining promises.

For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue.

The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement.

The Company’s arrangements may provide the collaboration partner with the right to select a target for licensing either at the inception of the arrangement or in the future. Under these arrangements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires.

For arrangements that include sales-based milestones and royalties, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any sales-based milestones or royalty revenue resulting from any of its arrangements.

Research and Development Services

The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under Exclusive Licenses above.

Reimbursements from the partner are evaluated as to whether the Company acts as a principal or an agent in such relationships. The Company evaluates whether control over the underlying goods or services were obtained prior to transferring these goods or services to the collaboration partner. Where the Company does not control the goods or services prior to transferring these goods or services to the collaboration partner, such reimbursements are presented net of costs.

At the inception of each arrangement that includes development milestone payments in respect of development efforts, the Company evaluates whether the development milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated development milestone value is included in the transaction price. Development milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular development milestone in making this assessment. There is judgment involved in determining whether it is probable that a significant revenue reversal would not occur.

At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of all development milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur.

Government Grants

The Company receives certain government and regional grants, which support its research efforts in defined projects, and include contributions towards the R&D cost. When there is reasonable assurance that the Company will comply with the conditions attached to a received grant, and when there is reasonable assurance that the grant will be received, government grants are recognized as revenue on a gross basis in the consolidated statement of profit or loss and comprehensive loss on a systematic basis over the periods in which the Company recognizes expenses for the related costs for which the grants are intended to compensate. In the case of grants related to assets, the received grant will be deducted from the carrying amount of the asset. Government grant revenue may be subject to review by a government authority in periods subsequent to their recognition and may result in the reversal of grant revenue previously recognized. Reversals of grant revenue are presented as contra revenue in the consolidated statement of operations.

Research and Development Expenses

Research and development expenses are expensed as incurred. Research and development expenses are comprised of costs incurred in providing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract research and development services, and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.

When third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history in conducting similar activities and the expected duration of the third-party service contract, among other considerations.

The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of research and development expenses.

The WBSO (afdrachtvermindering speur- en ontwikkelingswerk) is a Dutch fiscal facility that provides subsidies to companies, knowledge centers and self-employed people who perform research and development activities (as defined in the WBSO Act). Under this act, a contribution is paid towards the labor costs of employees directly involved in research and development. For the years ended December 31, 2021, 2020, and 2019, the Company recognized $9.3 million, $6.0 million and $4.5 million as a reduction of research and development expenses, respectively.

Share-Based Payments

The Company measures employee share-based compensation based on the grant date fair value of the share-based compensation award. The Company grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant, based on observable market prices.

For share-based payments subject time-based vesting, the Company recognizes employee stock-based compensation expense on a straight-line basis over the requisite service period of the awards, generally from the date of grant through each vesting date. The Company recognizes forfeitures at the time they occur. The actual expense recognized over the vesting period will only represent those options that vest; the effect of forfeitures in the recognition of periodic compensation expense are not estimated prior to their occurrence.

Earnings (Loss) per Share

The Company computes basic earnings (loss) per share by dividing income (loss) allocable to common stockholders by the weighted average number of shares of common stock outstanding. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities. During periods of loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted earnings (loss) per share after giving consideration to the dilutive effect of stock options and restricted stock units (“RSU”) that are outstanding during the period, except where such non-participating securities would be anti-dilutive.

Segment Information

The Company operates in one reportable segment, which comprises the discovery and development of innovative therapeutics.

Recently Adopted Accounting Pronouncements

In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The new guidance aligns the requirements for capitalizing implementation costs incurred in a cloud-based hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). This ASU became effective for the Company at the beginning of 2021, but had no impact on amounts or disclosures previously reported or during the year ended December 31, 2021.

In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808), which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue when the collaborative arrangement participant is a customer in the context of a unit of account and precludes recognizing as revenue consideration received from a collaborative arrangement participant if the participant is not a customer. The ASU became effective for the Company at the beginning of 2021. None of the Company’s arrangements fall within the scope of ASC 808, and the adoption of this standard had no impact on amounts or disclosures previously reported or during the year ended December 31, 2021.

Property and Equipment, Net - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 7,972	$ 7,608
Less: accumulated depreciation and amortization	(4,423)	(3,493)
Property and equipment, net	3,549	4,115
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and Equipment, gross	5,583	5,695
Office Equipment and Furniture
Property Plant And Equipment [Line Items]
Property and Equipment, gross	1,222	1,300
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and Equipment, gross	111	117
Construction in Progress
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 1,056	$ 496

Summary of Significant Accounting Policies - Summary of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2021
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Office Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	Shorter of useful life or term of lease

Employee Benefit Plans

12 Months Ended

Dec. 31, 2021

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plans

13. Employee Benefit Plans

Share-Based Payments

2010 Plan

In 2010, the Company established the Merus B.V. 2010 Employee Option Plan (the “2010 Plan”) that entitled key management personnel, staff and consultants providing similar services to purchase shares in the Company. Under the 2010 Plan, holders of vested options were entitled to purchase depositary receipts for common shares at the exercise price determined at the date of grant. Upon exercise of the option, common shares were issued to a foundation established to facilitate administration of share-based compensation awards and pool the voting interests of the underlying shares, and depositary receipts were issued by the foundation to the individual holders. In connection with the IPO, the 2010 Plan was amended to cancel the depositary receipts and allow individual holders to directly hold the common shares obtained upon exercise of their options.

Options granted under the 2010 Plan generally vest in installments over a four-year period from the grant date: 25% percent on the first anniversary of the vesting commencement date, and the remaining 75% of the options vest in 36 monthly installments for each full month of continuous service provided thereafter. Options expire after 8 years from the date of grant. The last grant of options pursuant to the 2010 Plan occurred in 2016, with no further grants expected.

2016 Plan

In 2016, the Company established the 2016 Incentive Award Plan (the “2016 Plan”). All incentive award grants since 2016 are being made under the 2016 Plan.

Options granted to employees under the 2016 Plan generally vest in installments over a four-year period from the grant date: 25% percent vest on the first anniversary of the vesting commencement date, and the remaining 75% of the options vest in 36 monthly installments for each full month of continuous service provided thereafter. Certain options may vest dependent on the attainment of performance criteria. Options expire after 10 years from the date of grant.

Options granted to non-executive directors consist of initial option grants as well as subsequent annual awards. The initial award of options granted vest in installments over a three-year period: 33% of the options vest on the first anniversary of the vesting commencement date, and 67% of the options vest in 24 monthly installments thereafter. Each subsequent award vests over a one-year period in 12 monthly installments. The Company measures the fair value of an option through the application of an option pricing model, as more fully described below.

The RSUs granted to employees under the 2016 Plan vest in installments over a four-year period from the grant date. Certain RSUs may vest dependent on the attainment of performance criteria. Each RSU represents the right to receive one common share. The fair value of an RSU is determined by reference to the price of the underlying common share.

The number of common shares authorized for issuance for future grants under the 2016 Plan as of January 1, 2022 totaled 2,836,603.

Share-Based Compensation Expense

Share-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:

	Year Ended December 31,
	2021		2020		2019
	(In thousands)
Research and development	$	7,167	$	2,969	$	3,186
General and administrative		9,924		6,403		4,648
Total	$	17,091	$	9,372	$	7,834

As of December 31, 2021, stock-based compensation expense related to unvested shares was $11.9 million. These shares are expected to vest and related costs are expected to be recognized over a weighted average remaining vesting period of 1.3 years.

Stock Option Valuation

The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. During the year ended December 31, 2020, the Company switched to using the Black-Scholes option-pricing model as part of its option plan administration system change. For the year ended December 31, 2019, a Hull-White option-pricing model was used. Key weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows:

	Year Ended December 31,
	2021			2020			2019
Risk-free interest rate		0.6	%		1.3	%		2.5	%
Contractual life of options (years)		10.0			10.0			10.0
Expected term of options (years)		6.1			6.3		N/A
Expected volatility of underlying stock		85.7	%		86.1	%		87.0	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The risk-free interest rate is based upon the U.S. Treasury yield curve in effect at the time of grant, with a term that approximates the expected life of the option. The Company determines the expected volatility using a blended approach encompassing its historical experience and the historical volatility of a peer group of comparable publicly traded companies with product candidates in similar stages of development to the Company’s product candidates. A simplified method using a weighted-average mid-point between an award’s vesting date and expiry is used to estimate the expected life of options in all periods presented as a sufficient history of participant exercise behavior is not readily observable. The Company has applied an expected dividend yield of 0.0% as the Company has not historically declared a dividend and does not anticipate declaring a dividend during the expected life of the options.

Stock Option Activity

The following is a summary of stock option activity for the year ended December 31, 2021:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(years)		(In thousands)
Outstanding at January 1, 2021		3,748,911		$	15.59
Granted		1,522,210			23.78
Exercised		(1,104,029	)		15.90
Forfeited or expired		(182,625	)		20.43
Outstanding at December 31, 2021		3,984,467		$	18.40		7.4	$	53,445
Exercisable at December 31, 2021		1,841,578		$	15.64		5.8	$	29,825

	Year Ended December 31,
	2021		2020
Weighted-average fair value of options granted	$	17.01	$	11.75

RSU Activity

The following is a summary of RSU activity for the year ended December 31, 2021:

	Number of RSUs			Weighted Average Grant-date Fair Value
Non-vested at January 1, 2021		59,775		$	16.48
Granted		24,976			23.40
Vested		(38,882	)		17.17
Forfeited		(35,869	)		17.15
Non-vested at December 31, 2021		10,000		$	25.56

Intrinsic Value of Stock Options Exercised and Vested RSUs

	Year Ended December 31,
	(In thousands)
	2021		2020
Total fair value of RSUs vested	$	903	$	961
Aggregate intrinsic value of options exercised		10,774		1,750

Post-Employment Benefit Plan

The Company has established a post-employment benefit plan for employees of the Netherlands that entitles executive officers and other staff members to retire at the age of 67 and receive annual payments based upon the average salary earned during the service period. The Company has insured the benefit liabilities through purchased non-participating annuities from an insurance company and has no other obligation other than to pay the annual insurance premiums to the insurance company. After purchasing the insurance, the Company has no further obligation (legal or constructive) to pay further amounts if the insurance fund has insufficient assets to pay all employee benefits relating to current and prior service. Contributions to purchase non-participating annuities are expensed as incurred as service costs. Company contributions to the post-employment benefit plan totaled $2.9 million, $2.0 million, and $1.3 million in the years ended December 31, 2021, 2020, and 2019, respectively.

401(k) Savings Plan

The Company has a defined contribution 401(k) savings plan (the “401(k) Plan”). The 401(k) Plan covers substantially all U.S. employees, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company matches contributions to the 401(k) Plan, matching 50% of an employee’s contribution up to a maximum of 3% of the participant’s compensation. Company contributions to the 401(k) Plan totaled $0.1 million for each of the years ended December 31, 2021, 2020, and 2019, respectively.

Executive Settlement

In December 2019, in connection with the departure of the Chief Executive Officer of the Company, the Company awarded benefits, including the following: cash compensation of $0.9 million, a grant of 30,000 RSUs, extended vesting of his equity incentive awards through June 30, 2021 and extended exercisability of his equity incentive awards through December 31, 2021. The cash compensation was paid by the Company in January 2020. There were no substantive service conditions associated with the benefits awarded other than the passage of time. The Company incrementally recognized $1.8 million in general and administrative expense associated with these benefits in the consolidated statement of operations for the year ended December 31, 2019.

In April 2020, Mark Throsby, Ph.D. resigned as the Executive Vice President and Chief Scientific Officer of the Company effective July 31, 2020. In connection with his departure, Mr. Throsby entered into a Settlement Agreement with the Company, pursuant to which Mr. Throsby received a severance payment equal to 8 months of his annual salary and amortized bonus aggregating approximately $0.3 million. Further, subject to Mr. Throsby’s continued compliance with the terms and conditions of the Settlement Agreement, Mr. Throsby’s unvested equity awards continued to vest until October 31, 2020 as if Mr. Throsby had continued in full time service with the Company through such date. The post-termination exercise period of Mr. Throsby's options was extended to March 31, 2021. The Company incrementally recognized $0.1 million in respect of the severance payment and a net reversal of $0.4 million of stock-based compensation expense in respect of share-based payments in research and development expense in the consolidated statement of operations in the prior year.

In March 2021, the Company and Mr. Throsby amended the Settlement Agreement, extending the post-termination expiration period of his outstanding options to extend to October 31, 2021, three months following his performance of certain consulting services through July 31, 2021. As a result, additional compensation cost of $0.2 million was recognized for the quarter ended March 31, 2021. During the three months ended September 30, 2021, the Company and Mr. Throsby entered into the 2^nd Amendment to the Settlement Agreement, extending Mr. Throsby’s consulting services period to November 30, 2021. The 2^nd Amendment extends the post-termination expiration period of his outstanding options to February 28, 2022. As the modification occurred in Mr. Throsby’s post-employment period, the options cease to be within the scope of ASC 718 and are recharacterized as an issuance of a standalone derivative instrument. The Company recognized a $1.0 million net loss associated with the derivative instrument included as other losses, net in the statement of operations for the year ended December 31, 2021.

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions								12 Months Ended
	Nov. 09, 2021	Jan. 21, 2021	Jan. 18, 2021	Dec. 03, 2020	Nov. 24, 2020	Nov. 23, 2020	Nov. 07, 2019	Dec. 31, 2021
Class Of Stock [Line Items]
Common stock voting rights								one vote
Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of common stock,net, shares							5,462,500
Issue price per share							$ 14.50
Net proceeds from public offering							$ 74.0
Number of common shares pursuant to underwriters' option to purchase additional shares							715,500
Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of common stock,net, shares	4,438,597	5,575,757	706,834	1,300,000	384,615	766,666
Issue price per share	$ 28.50	$ 24.75	$ 28.295	$ 16.90	$ 15.60	$ 15.00
Proceeds from issuance of common stock, net	$ 118.7	$ 129.4	$ 16.5	$ 22.0	$ 6.0	$ 11.5

Investments in Debt Securities

12 Months Ended

Dec. 31, 2021

Debt Securities [Abstract]

Investments in Debt Securities

3. Investments in Debt Securities

Debt securities are classified in the consolidated balance sheet as follows:

		December 31,
		2021		2020
		Balance		Balance
	(in thousands)
Cash equivalents		$	5,684	$	17,654
Current marketable securities			168,990		44,673
Non-current marketable securities			20,297		—
Total		$	194,971	$	62,327

The following table summarizes debt securities by maturity at December 31, 2021 (in thousands):

Maturity	Amortized Cost
Within one year	$	174,674
After one year through five years		20,297
Total	$	194,971

The following table summarizes debt securities by credit-quality indicator:

	Credit Quality Indicator as of December 31, 2021
	AAA		AA- to AA+		A- to A+		Total
	(In thousands)
Money market funds	$	5,684	$	—	$	—	$	5,684
Corporate paper and notes		—		28,898		126,141		155,039
U.S. government agency securities		—		2,093		—		2,093
U.S. treasuries		—		32,155		—		32,155
Total	$	5,684	$	63,146	$	126,141	$	194,971

The credit quality indicator was derived from publicly available ratings published by Moody’s or a comparable credit rating agency, last updated as of December 31, 2021.

The following table summarizes the fair value of debt securities by major security type held at December 31, 2021 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	5,684	$	—	$	—		$	5,684
Corporate paper and notes		155,039		—		(160	)		154,879
U.S. government agency securities		2,093		—		(7	)		2,086
U.S. treasuries		32,155		—		(31	)		32,124
Total	$	194,971	$	—	$	(198	)	$	194,773

The following table summarizes the fair value of debt securities by major security type held at December 31, 2020 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	10,156	$	—	$	—		$	10,156
Corporate paper and notes		27,978		2		—			27,980
U.S. government agency securities		9,150		—		—			9,150
U.S. treasuries		15,043		1		(2	)		15,042
Total	$	62,327	$	3	$	(2	)	$	62,328

The allowance for credit losses applicable to debt securities was immaterial in all periods presented.

Fair Value

The fair value of money market funds is determined based on publicly available market price for these funds (Level 1). The fair value of other debt securities is determined based on the publicly available inputs which includes a market price for the same or similar instruments adjusted for estimates in interest yield (Level 2).

Investments in Debt Securities (Tables)

12 Months Ended

Dec. 31, 2021

Debt Securities [Abstract]

Summary of Classification of Debt Securities in Consolidated Balance Sheet

Debt securities are classified in the consolidated balance sheet as follows:

		December 31,
		2021		2020
		Balance		Balance
	(in thousands)
Cash equivalents		$	5,684	$	17,654
Current marketable securities			168,990		44,673
Non-current marketable securities			20,297		—
Total		$	194,971	$	62,327

Summary of Debt Securities by Maturity

The following table summarizes debt securities by maturity at December 31, 2021 (in thousands):

Maturity	Amortized Cost
Within one year	$	174,674
After one year through five years		20,297
Total	$	194,971

Summary of Debt Securities by Credit Quality Indicator

The following table summarizes debt securities by credit-quality indicator:

	Credit Quality Indicator as of December 31, 2021
	AAA		AA- to AA+		A- to A+		Total
	(In thousands)
Money market funds	$	5,684	$	—	$	—	$	5,684
Corporate paper and notes		—		28,898		126,141		155,039
U.S. government agency securities		—		2,093		—		2,093
U.S. treasuries		—		32,155		—		32,155
Total	$	5,684	$	63,146	$	126,141	$	194,971

Summary of Fair Value of Debt Securities by Major Security Type

The following table summarizes the fair value of debt securities by major security type held at December 31, 2021 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	5,684	$	—	$	—		$	5,684
Corporate paper and notes		155,039		—		(160	)		154,879
U.S. government agency securities		2,093		—		(7	)		2,086
U.S. treasuries		32,155		—		(31	)		32,124
Total	$	194,971	$	—	$	(198	)	$	194,773

The following table summarizes the fair value of debt securities by major security type held at December 31, 2020 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	10,156	$	—	$	—		$	10,156
Corporate paper and notes		27,978		2		—			27,980
U.S. government agency securities		9,150		—		—			9,150
U.S. treasuries		15,043		1		(2	)		15,042
Total	$	62,327	$	3	$	(2	)	$	62,328

Income Taxes - Summary of Components of Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Net operating loss carryforwards	$ 78,709	$ 66,572
Deferred revenue	26,033	24,966
Excess interest carryforward	1,689	2,080
Lease obligation	1,006	1,058
Accrued expenses and other	478	494
Total deferred tax assets	107,915	95,170
Deferred tax asset valuation allowance	(106,440)	(93,645)
Total deferred tax assets, net of valuation allowance	1,475	1,525
Deferred tax liabilities:
Operating lease right-of-use assets	1,002	1,048
Other	56	67
Total deferred tax liabilities	1,058	1,115
Net deferred tax asset	$ 417	$ 410

Operating Leases (Tables)

12 Months Ended

Dec. 31, 2021

Leases [Abstract]

Summary of Components of Lease Cost

The components of lease cost recorded in the Company’s consolidated statement of operations and statement of cash flows were as follows:

	For the Year Ended December 31,
	2021		2020
	(In thousands)
Operating lease cost	$	1,693	$	1,611
Variable lease cost		408		375
Total lease cost included in operating expenses	$	2,101	$	1,986

Cash paid to lessors included in operating cash outflows	$	1,704	$	2,478

Schedule of Maturities of Operating Lease Obligations

Maturities of the Company’s operating lease obligations as of December 31, 2021 were as follows (in thousands):

Year	Operating Leases
2022	$	1,616
2023		880
2024		645
2025		662
2026		222
Total lease payments		4,025
Less: amount representing interest		(274	)
Total lease obligations	$	3,751

Schedule of Weighted Average Remaining Lease Terms and Discount Rates

The weighted-average remaining lease terms and discount rates related to the Company’s leases were as follows:

	As of December 31,
	2021			2020
Weighted-average remaining operating lease term (in years)		3.3			4.2
Weighted-average discount rate for operating leases		4.0	%		4.9	%

Accrued Expenses and Other Liabilities - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued research and development expenses	$ 15,174	$ 15,372
Accrued personnel costs	4,861	4,854
Accrued general and administrative expenses	1,362	1,566
Other	1,109	11
Accrued expenses	$ 22,506	$ 21,803

Income Taxes

12 Months Ended

Dec. 31, 2021

Income Tax Disclosure [Abstract]

Income Taxes

8. Income Taxes

The components of loss from operations before income tax expense are as follows:

	Year ended December 31,
	2021			2020			2019
				(In thousands)
United States	$	(8,502	)	$	(3,832	)	$	(1,363	)
Netherlands		(58,075	)		(81,178	)		(53,594	)
Total loss before income taxes	$	(66,577	)	$	(85,010	)	$	(54,957	)

The components of income tax expense (benefit) from continuing operations are as follows:

	December 31,
	2021			2020			2019
	(In thousands)
U.S. federal	$	173		$	391		$	243
U.S. state		73			234			40
Total current tax expense	$	246		$	625		$	283

U.S. federal	$	(5	)	$	(86	)	$	(63	)
U.S. state		(2	)		(36	)		(26	)
Total deferred tax benefit	$	(7	)	$	(122	)	$	(89	)

Total income tax expense	$	239		$	503		$	194

The Company recognizes income tax expense (benefit) based on its continuing operations in the U.S. The parent company in the Netherland has net operating losses.

The parent company is subject to income tax in the Netherlands where a greater proportion of economic activity is attributed. A reconciliation of the Netherlands statutory income tax rate to the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2021			2020			2019
Netherlands statutory income tax rate		25.0	%		25.0	%		25.0	%
Changes in tax rates		5.2			10.6			1.5
Non-deductible expenses		0.7			(2.4	)		(3.7	)
Change in valuation allowance		(31.4	)		(33.5	)		(23.3	)
Other		0.1			(0.3	)		0.1
Effective income tax rate		(0.4	)%		(0.6	)%		(0.4	)%

In 2020 and 2021, Dutch tax authorities enacted new tax rates applicable to future periods which impact the measurement of deferred income taxes. The effect of the change in the valuation allowance each year reflects the increase or decrease in the valuation allowance against deferred tax assets attributable to the Netherlands.

The components of the Company’s deferred tax assets (liabilities) consist of the following:

	December 31,
	2021			2020
	(In thousands)
Deferred tax assets:
Net operating loss carryforwards	$	78,709		$	66,572
Deferred revenue		26,033			24,966
Excess interest carryforward		1,689			2,080
Lease obligation		1,006			1,058
Accrued expenses and other		478			494
Total deferred tax assets		107,915			95,170
Deferred tax asset valuation allowance		(106,440	)		(93,645	)
Total deferred tax assets, net of valuation allowance		1,475			1,525

Deferred tax liabilities:
Operating lease right-of-use assets	$	1,002		$	1,048
Other		56			67
Total deferred tax liabilities		1,058			1,115

Net deferred tax asset	$	417		$	410

After consideration of all positive and negative evidence, the Company believes that it is more-likely-than-not that our Netherlands deferred tax assets that are not supported by reversing temporary differences will not be realized. As a result, the Company established a valuation allowance of $106.4 million and $93.6 million as of December 31, 2021 and 2020, respectively. The increase in the

valuation allowance of $12.8 million and $35.8 million for the years ended December 31, 2021 and 2020, respectively, is primarily attributable to the increase in net operating loss carryforward deferred tax assets for which a full valuation allowance applies and change in tax rates. As of December 31, 2021, the portion of the valuation allowance for deferred tax assets for which subsequently recognized tax benefits would be credited directly to contributed capital totaled $2.2 million.

As of December 31, 2021, the Company did not have any net operating losses for U.S. federal or state income tax purposes. The Company had net operating loss carryforwards for Dutch income tax purposes of $305.1 million as of December 31, 2021. Under Dutch tax law, net operating loss carryforwards may be used to offset future taxable income in full up to €1.0 million and 50% of taxable income that exceeds €1.0 million. Effective as of January 1, 2022, these losses can be carried forward indefinitely.

As of December 31, 2021, the Company had no unrecognized tax benefits. As of December 31, 2021, the Company had no accrued interest or penalties related to underpayments of income taxes and no amounts have been recognized in the consolidated statements of operations. The Company will recognize interest and penalties related to an underpayment of income taxes in income tax expense.

The Company files income tax returns in the U.S. federal and Massachusetts jurisdictions as well as in the Netherlands. The statute of limitations for assessment by the Internal Revenue Service (IRS), and Massachusetts tax authorities is closed for tax years prior to 2018. The statute of limitations for assessment by the Netherlands tax authorities is closed for tax years prior to 2016. The Company is not currently under examination by the IRS or any other jurisdictions for any tax years.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

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+ Details

Stockholders' Equity

12 Months Ended

Dec. 31, 2021

Equity [Abstract]

Stockholders' Equity

11. Stockholders’ Equity

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors.

Share Issuances

On November 7, 2019, the Company completed an underwritten public offering in which the Company sold 5,462,500 common shares, including 715,500 common shares pursuant to the underwriters’ option to purchase additional shares, at a price to the public of $14.50 for aggregate net proceeds of $74.0 million.

On November 23, 2020, the Company sold 766,666 common shares, at a price of $15.00 for aggregate proceeds of $11.5 million.

On November 24, 2020, the Company sold 384,615 common shares, at a price of $15.60 for aggregate proceeds of $6.0 million.

On December 3, 2020, the Company sold 1,300,000 common shares, at a price of $16.90 for aggregate proceeds of $22.0 million.

On January 18, 2021, the Company sold 706,834 common shares, at a price of $28.295 for aggregate proceeds of $16.5 million.

On January 21, 2021, the Company sold 5,575,757 common shares, at a price of $24.75 for aggregate proceeds of $129.4 million.

On November 9, 2021, the Company sold 4,438,597 common shares, at a price of $28.50 for aggregate proceeds of $118.7 million.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

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+ Details

Intangible Assets, Net (Tables)

12 Months Ended

Dec. 31, 2021

Intangible Assets Gross Excluding Goodwill [Abstract]

Summary of Intangible Assets, Net

Intangible assets, net consists of the following:

	December 31,
	2021			2020
	(In thousands)
Licenses of intellectual property	$	3,597		$	3,898
Software licenses		266			288
Intangible assets		3,863			4,186
Less: accumulated amortization		(1,516	)		(1,343	)
Intangible assets, net	$	2,347		$	2,843

Schedule of Expected Amortization Expense

Amortization expense over the next five years are expected to be as follows (in thousands):

Year	Expected amortization
2022	$	276
2023		213
2024		181
2025		154
2026		154
Thereafter		1,369
Total remaining value	$	2,347

Intangible Assets, Net

12 Months Ended

Dec. 31, 2021

Goodwill And Intangible Assets Disclosure [Abstract]

Intangible Assets, Net

6. Intangible assets, net

Intangible assets, net consists of the following:

	December 31,
	2021			2020
	(In thousands)
Licenses of intellectual property	$	3,597		$	3,898
Software licenses		266			288
Intangible assets		3,863			4,186
Less: accumulated amortization		(1,516	)		(1,343	)
Intangible assets, net	$	2,347		$	2,843

Amortization expense was $0.2 million, $0.3 million and $0.2 million for the years ended December 31, 2021, 2020, and 2019, respectively. Intangible assets are predominantly located in the Netherlands.

Amortization expense over the next five years are expected to be as follows (in thousands):

Year	Expected amortization
2022	$	276
2023		213
2024		181
2025		154
2026		154
Thereafter		1,369
Total remaining value	$	2,347

Accrued Expenses and Other Liabilities

12 Months Ended

Dec. 31, 2021

Payables And Accruals [Abstract]

Accrued Expenses and Other Liabilities

7. Accrued Expenses and Other Liabilities

Accrued expenses consisted of the following:

	December 31,
	2021		2020
	(In thousands)
Accrued research and development expenses	$	15,174	$	15,372
Accrued personnel costs		4,861		4,854
Accrued general and administrative expenses		1,362		1,566
Other		1,109		11
Accrued expenses	$	22,506	$	21,803

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 241,435	$ 163,082
Marketable securities	168,990	44,673
Accounts receivable	1,697	46
Accounts receivable (related party)	4,609	1,623
Prepaid expenses and other current assets	7,448	8,569
Total current assets	424,179	217,993
Marketable securities	20,297
Property and equipment, net	3,549	4,115
Operating lease right-of-use assets	3,733	3,907
Intangible assets, net	2,347	2,843
Deferred tax assets	417	410
Other assets	2,078	1,949
Total assets	456,600	231,217
Current liabilities:
Accounts payable	13,237	3,126
Accrued expenses and other liabilities	22,506	21,803
Income taxes payable		206
Current portion of lease obligation	1,494	1,432
Current portion of deferred revenue	16,613	625
Current portion of deferred revenue (related party)	18,048	19,554
Total current liabilities	71,898	46,746
Lease obligation	2,257	2,521
Deferred revenue, net of current portion	10,962	237
Deferred revenue, net of current portion (related party)	55,282	79,450
Total liabilities	140,399	128,954
Commitments and contingencies (Note 10)
Stockholders’ equity:
Common shares, €0.09 par value; 67,500,000 and 45,000,000 shares authorized as at December 31, 2021 and 2020, respectively; 43,467,052 and 31,602,953 shares issued and outstanding as at December 31, 2021 and 2020, respectively	4,481	3,211
Additional paid-in capital	787,869	490,093
Accumulated deficit	(466,928)	(400,112)
Accumulated other comprehensive (loss) income	(9,221)	9,071
Total stockholders’ equity	316,201	102,263
Total liabilities and stockholders’ equity	$ 456,600	$ 231,217

Income Taxes - Reconciliation of Netherlands Statutory Income Tax Rate and Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Netherlands statutory income tax rate	25.00%	25.00%	25.00%
Changes in tax rates	5.20%	10.60%	1.50%
Non-deductible expenses	0.70%	(2.40%)	(3.70%)
Change in valuation allowance	(31.40%)	(33.50%)	(23.30%)
Other	0.10%	(0.30%)	0.10%
Effective income tax rate	(0.40%)	(0.60%)	(0.40%)

Income Taxes - Additional Information (Details) € in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2021 EUR (€)	Dec. 31, 2020 USD ($)
Income Tax Disclosure [Line Items]
Deferred tax asset valuation allowance	$ 106,440,000		$ 93,645,000
Increase in valuation allowance, attributable to increase in net operating loss carryforward deferred tax assets	12,800,000		$ 35,800,000
Deferred tax asset valuation allowance	2,200
Unrecognized tax benefits	0
Accrued interest or penalties related to uncertain tax positions	0
Unrecognized tax benefits, tax related penalties and interest expense	0
Dutch Tax Law
Income Tax Disclosure [Line Items]
Operating Loss Carryforwards	$ 305,100,000
Taxable income that exceeds €1.0 million, percentage	50.00%	50.00%
Dutch Tax Law \| Maximum
Income Tax Disclosure [Line Items]
Future taxable income offset limit \| €		€ 1.0
Dutch Tax Law \| Minimum
Income Tax Disclosure [Line Items]
Future taxable income exceeding limit \| €		€ 1.0

Collaborations - Additional Information (Details) $ / shares in Units, $ in Thousands, € in Millions			1 Months Ended		12 Months Ended
	Jan. 18, 2021 USD ($) $ / shares shares	Mar. 14, 2018 EUR (€) Antibody	Jan. 31, 2018 USD ($) Antibody PerformanceObligation	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) Milestone	Dec. 31, 2020 USD ($) Milestone	Dec. 31, 2019 USD ($) Milestone	Dec. 31, 2019 EUR (€) Milestone	Dec. 31, 2018 USD ($)	Feb. 12, 2021 USD ($)	Dec. 10, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net fair value of shares					$ 285
Milestones achieved recognized					2,000
Development milestones received					1,000		$ 4,100	€ 3.7
Number of milestones achieved \| Milestone						0	0	0
Milestone payments received						$ 500
Research and development					98,187	70,040	$ 55,680
Lilly Collaborations Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable	$ 40,000
Issuance of common stock,net, shares \| shares	706,834
Common stock stated purchase price per share \| $ / shares	$ 28.295
Proceeds from issuance of common stock, net	$ 20,000
Fixed consideration	43,500									$ 43,500
Upfront payment	40,000
Net fair value of shares	$ 16,500
Incyte Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable				$ 120,000
Issuance of common stock,net, shares \| shares				3,200,000
Common stock stated purchase price per share \| $ / shares				$ 25.00
Collaborations, transaction price as revenue				$ 80,000
Incyte Collaboration and License Agreement \| License and Related Activities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborations, transaction price as revenue				$ 152,600
Second ONO Research and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable \| €		€ 0.7
Payment to compensate research services \| €		0.3
Over time payment for full time equivalent funding \| €		€ 0.2
Second ONO Research and License Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of bispecific antibodies under collaboration and license agreement \| Antibody		5
Simcere Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborations, transaction price as revenue			$ 2,750
Number of milestones achieved \| Milestone							0	0
Non-refundable upfront payment received			$ 2,750
Milestone payments received					$ 500
Simcere Collaboration and License Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of bispecific antibodies under collaboration and license agreement \| Antibody			3
Simcere Collaboration and License Agreement \| License and Performance \| Research and Development
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of performance obligation \| PerformanceObligation			3
Betta Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment receivable											$ 1,000
Development milestones received						2,000
Number of milestones achieved \| Milestone					0		0	0
Contingent milestone payments receivable											$ 12,000
Revenue related performance obligation as revenue									$ 1,000
Milestone revenue						2,000
Research and development						$ 2,000

Employee Benefit Plans - Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total	$ 17,091	$ 9,372	$ 7,834
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total	7,167	2,969	3,186
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total	$ 9,924	$ 6,403	$ 4,648

CONSOLIDATED BALANCE SHEETS (Parenthetical) - € / shares	Dec. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Common shares, par value	€ 0.09	€ 0.09
Common shares, authorized	67,500,000	45,000,000
Common shares, issued	43,467,052	31,602,953
Common shares, outstanding	43,467,052	31,602,953

Subsequent Events - Additional Information (Details)	1 Months Ended
Subsequent Events - Additional Information (Details)	Jan. 31, 2022
Subsequent Event \| Maximum \| Incyte Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Residual royalty percentage	4.00%

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Total revenue	$ 49,107	$ 29,943	$ 31,133
Operating expenses:
Research and development	98,187	70,040	55,680
General and administrative	40,896	35,781	34,110
Total operating expenses	139,083	105,821	89,790
Operating loss	(89,976)	(75,878)	(58,657)
Other income (loss), net:
Interest (expense) income, net	(129)	300	1,889
Foreign exchange (losses) gains, net	24,663	(9,432)	1,615
Other (losses) gains, net	(1,135)	0	196
Total other income (loss), net	23,399	(9,132)	3,700
Loss before income tax expense	(66,577)	(85,010)	(54,957)
Income tax expense	239	503	194
Net loss	(66,816)	(85,513)	(55,151)
Other comprehensive income (loss):
Currency translation adjustment	(18,292)	7,485	(1,308)
Comprehensive loss	$ (85,108)	$ (78,028)	$ (56,459)
Net loss per share allocable to common stockholders:
Basic and diluted	$ (1.73)	$ (2.92)	$ (2.28)
Weighted-average common shares outstanding:
Basic and diluted	38,638,434	29,256,203	24,218,083
Collaboration Revenue [Member]
Total revenue from contracts with customers	$ 19,503	$ 3,363	$ 5,517
Collaboration Revenue (Related Party) [Member]
Total revenue from contracts with customers	29,604	26,580	25,831
Grant Revenue [Member]
Total revenue from contracts with customers	$ 0	$ 0	$ (215)

Collaborations

12 Months Ended

Dec. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaborations

12. Collaborations

Lilly

On January 18, 2021, Eli Lilly and Company (“Lilly”) agreed to pay the Company a $40.0 million, non-refundable upfront payment, and purchased 706,834 common shares at a stated price per share of $28.295, for an aggregate purchase price of $20.0 million. The Company and Lilly agreed to collaborate with respect to the discovery and research of bispecific antibodies utilizing the Company’s proprietary Biclonics® bispecific technology platform. The collaboration encompasses up to three (3) independent programs directed to the generation of T-cell re-directing bispecific antibodies that bind CD3 and a tumor associated antigen target selected by Lilly to be the subject of each program.

The objective of each program is to develop a lead compound that Lilly would be able to continue to develop through clinical trials. Lilly agreed to fund the research activities the Company conducts for each program under an agreed research plan and budget. Lilly receives an exclusive, worldwide, royalty-bearing, sublicensable license, under certain patent rights and know-how to exploit certain compounds and products directed to designated targets in combination with targeting CD3, or directed to such designated target(s) alone as a monospecific antibody or monospecific antibody drug conjugate, subject to rights granted by Merus to third parties under one or more existing third party agreements. Merus retains all rights not granted to Lilly. Lilly has certain rights to replace selected targets, including the right to substitute a target selection after initial selection for a period of time. The Company may be entitled to further milestones and royalties in the future dependent on development and commercialization of any resulting product.

The initial term of the arrangement includes a three-year research term for the Company to perform research and development activities, subject to two extension terms of six months at Lilly’s discretion. While the arrangement may be terminated in its entirety or on a program-by-program basis at will by Lilly, there are no direct costs or penalties to Lilly to terminate the arrangement prior to the end of the initial term.

At inception of the arrangement, the Company identified a single performance obligation comprised of a combined delivery of a license and related activities, including research activities associated with a product candidate against the first target and the activities of the joint steering committee. The Company also identified two other combined performance obligations relating to options exercisable by Lilly to select a second and third target to advance a second and third product candidate against the selected targets through discovery and research.

The transaction price at inception was comprised of fixed consideration of $43.5 million that was derived from the $40.0 million upfront payment and $20.0 million share purchase proceeds, net of the fair value of shares of the shares delivered to Lilly of $16.5 million, and variable consideration associated with the funding of research services for the product candidate against the first target at inception. All other consideration under the arrangement was determined to be variable consideration and fully constrained at inception.

The fixed consideration was allocated equally amongst the three performance obligations and the variable consideration associated with each target was allocated to the performance obligation of each respective target. The equal allocation of the fixed consideration was based on the estimated standalone selling price of each performance obligation as each was materially the same.

On February 12, 2021, the Company and Lilly completed the initial exchange of fixed consideration and transfer of common shares. The Company initially deferred $43.5 million allocated to the performance obligations to be recognized as revenue over time using a cost-to-cost measure of progress toward the development of a lead compound for each respective target, anticipated to be recognized as revenue within the initial research term, along with research funding. Development milestones, commercialization milestones and royalties are variable consideration, fully constrained, to be included in the transaction price for each performance obligation and recognized in future periods in accordance with the Company’s revenue recognition policy. The revenue recognized relating to each combined performance obligation is presented in the notes according to the source of consideration received (upfront, reimbursement revenue, milestone), reflective of their differing timing of receipt.

Incyte

In December 2016, pending regulatory clearance, Incyte Corporation (“Incyte”) agreed to pay the Company a $120.0 million, non-refundable upfront payment, and purchased 3.2 million common shares at a stated price per share of $25.00, for an aggregate purchase price of $80.0 million. In exchange, the Company granted Incyte with a license to certain of its intellectual property and committed to collaborate with Incyte to research, discover and develop monospecific or bispecific antibodies utilizing the Company’s proprietary bispecific technology platform. The collaboration is managed by a joint steering committee in which both parties are represented and is tasked with overseeing the activities which significantly contributes to the collaboration. The collaboration may encompass up to 10 product candidates that result from the Company’s application of its proprietary Biclonics^® technology platform. During the course of the initial research term, Merus proposes product candidates to Incyte, which evaluates whether to designate proposed product candidates from the Company to make a selection for further research. Proposed product candidates begin at a pre-clinical stage of development. Incyte has certain rights to replace product candidates, including the right to substitute a product candidate after initial selection. The Company would be entitled to future consideration in the form of cost reimbursements for research services, development milestones, commercialization milestones and royalties related to the programs under the arrangement.

At inception of the collaboration, two potential bispecific product candidates were under preliminary evaluation. After further research, a lead candidate was ultimately selected for the first product candidate, designated MCLA-145, and the other potential product candidate was not pursued. For the designated product candidate (MCLA-145), the Company retained the exclusive right to develop and commercialize products and product candidates in the United States, while Incyte obtained the exclusive right to develop and commercialize products and product candidates arising from such program outside the United States. For MCLA-145, the parties will conduct and share equally the costs of mutually agreed global development activities and will be solely responsible for independent development activities in each party’s respective territories. For all other programs under the arrangement to be selected by Incyte, Incyte will be responsible for all research, development and commercialization costs. The Company may elect to co-fund the development of certain of the other programs in the future, in which case costs and benefits would be shared. The Company has not elected to co-fund any programs to date.

At inception of the arrangement, the Company identified a performance obligation comprised of a combined delivery of a license and related activities, including the activities of the joint steering committee, to which to allocate consideration. The arrangement also allowed for optional future research services to advance selected product candidates through discovery and research. The transaction price was comprised of fixed consideration of an upfront payment of $120.0 million and proceeds from the sale of shares of $80.0 million. All other consideration under the arrangement was determined to be variable consideration and fully constrained at inception. $152.6 million of the transaction price was allocated to the license and related activities performance obligation after accounting for the purchase of common shares by Incyte.

On January 23, 2017, the Company completed the sale of shares and exchange of the license. The Company initially deferred the transaction price allocated to the license and related activities performance obligation as deferred revenue, to be recognized as revenue over time as the primary benefit of the license to Incyte is access to the platform for the generation of potential product candidates. Development milestones, commercialization milestones and royalties are variable consideration, fully constrained, to be recognized in future periods in accordance with the Company’s revenue recognition policy. Cost reimbursements for research services are recognized as they are performed over time as these are considered a separate performance obligation.

As of December 31, 2021, the Company is currently engaged in clinical development activities for MCLA-145 and developing pre-clinical candidates for the other programs. Refer to Note 16, “Subsequent Events.” During the year ended December 31, 2021, the Company recognized a total of $2.0 million for achieving development milestones and received a $1.0 million payment. There were no development milestones achieved for the years ended December 31, 2020 and 2019.

ONO

In April 2014, the Company granted ONO Pharmaceuticals Co. Ltd. (“ONO”) an exclusive, worldwide, royalty-bearing license, with the right to sublicense, research, test, make, use and market a limited number of bispecific antibody candidates based on the Company’s Biclonics^® technology platform against two undisclosed targets directed to a particular undisclosed target combination.

On March 14, 2018, the Company granted ONO an exclusive, worldwide, royalty-bearing license, with the right to sublicense, research, test, make, use and market a limited number of bispecific antibody candidates based on the Company’s Biclonics^® technology platform against two undisclosed targets directed to a particular undisclosed target combination. ONO is responsible for identifying lead candidates and conducting further non-clinical and clinical development activities for such licensed bispecific antibodies and pharmaceutical products containing such antibodies, including manufacture and process development. Additionally, ONO controls and has exclusive rights over the worldwide commercialization of any approved products, including worldwide supply, and is solely responsible for all costs and expenses related to commercialization. ONO has also agreed to fund the Company’s research and development activities and be responsible for the payment of all costs and expenses for its own research and development activities, which are set out in a mutually agreed upon research plan. The Company retains all rights to use and commercialize any antibodies that are generated under the collaborative research program, excluding the up to five lead and/or selected antibodies against the targets ONO is pursuing, provided that the use and commercialization is not with respect to the particular target combination. ONO agreed to pay the Company an upfront, non-refundable payment of €0.7 million. In addition, the Company was entitled to €0.3 million intended to compensate the Company for research services already completed upon entering into the agreement, and €0.2 million to be paid to the Company over time for full time equivalent funding. The Company is entitled to research and development milestones in addition to royalties on future sales.

The Company identified performance obligations for: (1) provision of a license for the target combination, and (2) research and development services. The Company concluded that Ono would be able to develop and benefit from the license, independent of the research and development services. The research and development services are capable of being performed by third parties with an appropriate sub-license, and are recognized over time as these services are delivered. Milestone payments are fully constrained as variable consideration to be recognized in future periods in accordance with the Company’s revenue recognition policy.

The Company received €3.7 million (approximately $4.1 million) for the year ended December 31, 2019 for development milestones. There were no development milestones achieved in the years ended December 31, 2021 and 2020.

Simcere

In January 2018, the Company granted Simcere Pharmaceuticals Group (“Simcere”) an exclusive license to develop and commercialize up to three bispecific antibodies to be produced by Merus utilizing the Company’s Biclonics^® technology platform in China (the “Simcere Agreement”). The Company will retain all rights outside of China. The Company has agreed to lead research and discovery activities, while Simcere has agreed to be responsible for the Investigational New Drug (“IND”) enabling studies, clinical development, regulatory filings and commercialization of these potential product candidates in China. The Company received an upfront, non-refundable payment of $2.75 million, relating to three separate research programs. Refer to Note 16, “Subsequent Events.”

At inception of the arrangement, the Company identified three performance obligations comprised of the combined delivery of a license and performance of research and development activities with respect to each program. The Company performs research and development activities to achieve candidate nomination. The Company concluded that these activities were not distinct from the underlying license for each program as Simcere would not be able to benefit from the license apart from research and development activities at this phase of development.

The transaction price under the arrangement comprised fixed consideration of $2.75 million. The transaction price was allocated to each separate performance obligation on a relative standalone fair value basis. The Company deferred the portion of the upfront payment allocated to the three performance obligations as deferred revenue, to be recognized over time. Compensation for research and development services prior to candidate nomination are allocated to each program performance obligation and also recognized over time. Development milestone payments allocated to each of the program performance obligations are constrained as variable consideration to be recognized in future periods in accordance with the Company’s revenue recognition policy.

During the years ended December 31, 2021 and 2020, the Company has recognized $0.5 million for milestones achieved under the Simcere Agreement, respectively. There was no milestone achieved during the year ended December 31, 2019.

Betta

On December 10, 2018, the Company granted Betta an exclusive license to develop and commercialize in China MCLA-129, proprietary Biclonics® produced by its Biclonics® technology platform. The Company retains all rights outside of China. Betta has agreed to retain a contract manufacturing organization with experience in filing IND applications with U.S. regulatory authorities and CTAs with European regulatory authorities in order to produce clinical trial materials for the Chinese market and rest of the world. As a key strategic component of the collaboration, Betta will be responsible for IND enabling studies and manufacturing of clinical trial materials in China, which the Company intends to use to assist regulatory filing and early stage clinical development in the rest of the world.

In addition to a non-refundable upfront payment of $1.0 million, Betta and the Company will share equally the cost of the transfer of the manufacturing technology to a contract manufacturing organization. The Company is also eligible to receive an aggregate of $12.0 million in milestone payments contingent upon Betta achieving certain specified development and commercial goals as well as tiered royalty payments of net sales of any products resulting from the collaboration in China. In turn, Betta is also entitled to milestone payments based on the Company’s progress.

The Company identified a single combined performance obligation, being the delivery of the MCLA-129 license including activities necessary to complete the technology transfer. The Company had no other commitments. The transaction price is comprised of fixed consideration of $1.0 million and fully allocated to the single performance obligation which would be fulfilled at a point in time. The technology transfer to deliver the license was completed in 2018 and Company recognized the revenue related to this performance

obligation of $1.0 million as revenue for the year ended December 31, 2018. Development milestone payments allocated to the performance obligation are constrained as variable consideration to be recognized in future periods in accordance with the Company’s revenue recognition policy.

During the year ended December 31, 2020, both the Company and Betta achieved a development milestone both valued at $2.0 million. The amounts are recognized as milestone revenue of $2.0 million and research and development cost of $2.0 million in the Company’s statement of operations for the year ended December 31, 2020. No milestones were achieved during the years ended December 31, 2021 and 2019, respectively.

Contract Assets, Liabilities, Revenues and Expenses

The following tables provide amounts by year indicated and by line item included in the Company's accompanying consolidated financial statements attributable to transactions arising from its collaboration arrangements. The dollar amounts in the tables below are in thousands.

	Related Party			Third Party
	Incyte			Lilly			Other			Total
Contract assets
Accounts receivable
Balance at January 1, 2021	$	—		$	—		$	46		$	46
Billings		8,053			62,061			939			63,000
Cash receipts		(6,419	)		(62,061	)		(924	)		(62,985	)
Adjustments		—			—						—
Foreign exchange		—			—			(20	)		(20	)
Balance at December 31, 2021		1,634			—			41			41

Unbilled receivables
Balance at January 1, 2021	$	1,623		$	—		$	—		$	—
Accrued receivables		9,361			3,264			1,408			4,672
Billings		(8,030	)		(2,061	)		(939	)		(3,000	)
Adjustments		21			—			—			—
Foreign exchange		—			—			(16	)		(16	)
Balance at December 31, 2021		2,975			1,203			453			1,656

Contract liabilities
Deferred revenue
Balance at January 1, 2021	$	99,004		$	—		$	862		$	862
Allocation of contract consideration		—			43,523			—			43,523
Revenue recognized in the period		(18,864	)		(14,012	)		(602	)		(14,614	)
Foreign exchange		(6,810	)		(2,158	)		(38	)		(2,196	)
Balance at December 31, 2021		73,330			27,353			222			27,575
Less: current portion		(18,048	)		(16,391	)		(222	)		(16,613	)
Non-current balance at December 31, 2021		55,282			10,962			—			10,962

The balance of unbilled receivables predominantly represents reimbursement revenue under the Company’s collaboration arrangements earned in the period to be billed and collected in the next period, generally quarterly. Incyte is a related party as a shareholder, as more fully described in Note 15.

	For the Year Ended December 31, 2021
	Related Party		Third Party
	Incyte		Lilly		Other			Total
Upfront payments	$	18,864	$	14,012	$	602		$	14,614
Reimbursement revenue		8,740		3,332		1,057			4,389
Milestones		2,000		—		500			500
Total collaboration revenue	$	29,604	$	17,344	$	2,159		$	19,503

Operating expenses:
Research and development expense	$	1,223	$	—	$	151		$	151
General and administrative expense		—		—		—			—
Total operating expenses from collaborations	$	1,223	$	—	$	151		$	151

Revenue recognized that was included in deferred revenue at the beginning of the period	$	18,864	$	—	$	602		$	602

	For the Year Ended December 31, 2020
	Related Party		Third Party
	Incyte		Lilly		Other			Total
Upfront payments	$	18,193	$	—	$	895		$	895
Reimbursement revenue		8,387		—		(12	)		(12	)
Milestones		—		—		2,480			2,480
Total collaboration revenue	$	26,580	$	—	$	3,363		$	3,363

Operating expenses:
Research and development expense	$	2,036	$	—	$	1,944		$	1,944
General and administrative expense		—		—		—			—
Total operating expenses from collaborations	$	2,036	$	—	$	1,944		$	1,944

Revenue recognized that was included in deferred revenue at the beginning of the period	$	18,193	$	—	$	938		$	938

	For the Year Ended December 31, 2019
	Related Party		Third Party
	Incyte		Lilly		Other			Total
Upfront payments	$	17,839	$	—	$	839		$	839
Reimbursement revenue		7,992		—		252			252
Milestones		—		—		4,426			4,426
Total collaboration revenue	$	25,831	$	—	$	5,517		$	5,517

Operating expenses:
Research and development expense	$	680	$	—	$	—		$	—
General and administrative expense		—		—		—			—
Total operating expenses from collaborations	$	680	$	—	$	—		$	—

Revenue recognized that was included in deferred revenue at the beginning of the period	$	17,839	$	—	$	1,141		$	1,141

Prepaid Expenses and Other Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical and manufacturing costs	$ 2,146	$ 4,971
Prepaid general and administrative costs	2,760	2,460
Interest receivable	367	80
Other	2,175	1,058
Total	$ 7,448	$ 8,569

Loss per Share

12 Months Ended

Dec. 31, 2021

Earnings Per Share [Abstract]

Loss per Share

14. Loss per Share

The two-class method was not applied for the years ended December 31, 2021, 2020 and 2019 due to the net loss recognized in each of those periods.

Basic and diluted loss per share allocable to common stockholders are computed as follows:

	Year Ended December 31,
	2021			2020			2019
	(In thousands except per share data)
Net loss	$	(66,816	)	$	(85,513	)	$	(55,151	)
Weighted average shares outstanding		38,638,434			29,256,203			24,218,083
Basic and diluted loss per share allocable to common stockholders	$	(1.73	)	$	(2.92	)	$	(2.28	)

Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2021

Compensation And Retirement Disclosure [Abstract]

Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss

Share-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows:

	Year Ended December 31,
	2021		2020		2019
	(In thousands)
Research and development	$	7,167	$	2,969	$	3,186
General and administrative		9,924		6,403		4,648
Total	$	17,091	$	9,372	$	7,834

Summary of Key Weighted Average Assumptions Used in Pricing Model on Date of Grant for Options Granted to Employees

	Year Ended December 31,
	2021			2020			2019
Risk-free interest rate		0.6	%		1.3	%		2.5	%
Contractual life of options (years)		10.0			10.0			10.0
Expected term of options (years)		6.1			6.3		N/A
Expected volatility of underlying stock		85.7	%		86.1	%		87.0	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Stock Option Activity

The following is a summary of stock option activity for the year ended December 31, 2021:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(years)		(In thousands)
Outstanding at January 1, 2021		3,748,911		$	15.59
Granted		1,522,210			23.78
Exercised		(1,104,029	)		15.90
Forfeited or expired		(182,625	)		20.43
Outstanding at December 31, 2021		3,984,467		$	18.40		7.4	$	53,445
Exercisable at December 31, 2021		1,841,578		$	15.64		5.8	$	29,825

	Year Ended December 31,
	2021		2020
Weighted-average fair value of options granted	$	17.01	$	11.75

Summary of RSU Activity

The following is a summary of RSU activity for the year ended December 31, 2021:

	Number of RSUs			Weighted Average Grant-date Fair Value
Non-vested at January 1, 2021		59,775		$	16.48
Granted		24,976			23.40
Vested		(38,882	)		17.17
Forfeited		(35,869	)		17.15
Non-vested at December 31, 2021		10,000		$	25.56

Intrinsic Value of Stock Options Exercised and Vested RSUs

	Year Ended December 31,
	(In thousands)
	2021		2020
Total fair value of RSUs vested	$	903	$	961
Aggregate intrinsic value of options exercised		10,774		1,750

Investments in Debt Securities - Summary of Debt Securities by Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Held To Maturity Securities Debt Maturities Single Maturity Date Amortized Cost Basis [Abstract]
Within one year	$ 174,674
After one year through five years	20,297
Total	$ 194,971	$ 62,327

Related Party Transactions - Additional Information (Details) - Incyte - Merus N V [Member]	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Ownership percentage	8.20%	10.10%	11.10%
Maximum
Related Party Transaction [Line Items]
Ownership percentage	10.00%

Prepaid Expenses and Other Assets - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Other Assets
Restricted cash held on account as collateral	$ 0.3	$ 0.2

Intangible Assets, Net - Schedule of Expected Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Finite Lived Intangible Assets Future Amortization Expense Current And Five Succeeding Fiscal Years [Abstract]
2022	$ 276
2023	213
2024	181
2025	154
2026	154
Thereafter	1,369
Intangible assets, net	$ 2,347	$ 2,843

Related Party Transactions

12 Months Ended

Dec. 31, 2021

Related Party Transactions

15. Related Party Transactions

The Company has entered into the Incyte collaboration and license agreement and the Incyte share subscription agreement in which the terms and transactional amounts incurred between Incyte and the Company are more fully described in Note 12. Incyte is a shareholder with holdings representing approximately 8.2%, 10.1% and 11.1% of the outstanding shares of the Company as of December 31, 2021, 2020 and 2019, respectively. During the year ended December 31, 2021, Incyte’s holdings of the Company’s outstanding shares fell below 10.0% of the Company’s total outstanding shares due to the Company issuing additional shares of common stocks through various financing events of 2021. These consolidated financial statements present Incyte as a related party in order to simplify the presentation and clearly display transactional amounts incurred between Incyte and the Company, given the related party relationship in effect for a portion of the year.

Operating Leases - Summary of Components of Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Lease Cost [Abstract]
Operating lease cost	$ 1,693	$ 1,611
Variable lease cost	408	375
Total lease cost included in operating expenses	2,101	1,986
Cash paid to lessors included in operating cash outflows	$ 1,704	$ 2,478

Subsequent Events

12 Months Ended

Dec. 31, 2021

Subsequent Events [Abstract]

Subsequent Events

16. Subsequent Events

In January 2022, the Company and Simcere agreed to terminate the Simcere Agreement, effective March 30, 2022 .

In January 2022, the Company announced that Incyte elected to opt-out of its ex-U.S. development of MCLA-145, from the parties joint collaboration agreement executed in 2017. At inception of the collaboration, for the designated product candidate (MCLA-145), the Company retained the exclusive right to develop and commercialize products and product candidates in the United States, while Incyte obtained the exclusive right to develop and commercialize products and product candidates arising from such program outside the United States. For MCLA-145, the parties conducted and shared equally the costs of mutually agreed global development activities. Incyte’s opt-out of ex-U.S. rights to MCLA-145 provides the Company the exclusive right to develop and commercialize potential MCLA-145 products globally. Under the collaboration, Incyte will continue to support the program for a limited time while ex-U.S. activities are transitioned to the Company, and Incyte will also retain a right to a residual royalty of up to 4% on sales of future commercialization of MCLA-145, if approved.

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Netherlands
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 1.2	$ 1.2	$ 0.9

Operating Leases

12 Months Ended

Dec. 31, 2021

Leases [Abstract]

Operating Leases

9. Operating Leases

Merus N.V. has non-cancellable operating leases for its corporate headquarters in Utrecht, the Netherlands. During the year ended December 31, 2021, the Company signed a lease amendment (the “Amendment”), which extended the lease term by approximately 1.5 years ending at the end of March 2023. The Amendment did not include additional right-of-use other than the extended lease term. There is no additional renewal term included in the Amendment to consider in the estimate of the lease term.

In March 2019, Merus US, Inc. entered into a non-cancellable operating lease agreement for office space in Cambridge, Massachusetts. The lease commenced in the second quarter of 2019 and has a term of seven years, and may be extended for another five years. Given the Company’s current plans, the renewal term has not been included in the estimate of the lease term. Fixed lease payments increase annually and include an increase on an inflationary measure. Variable payments include amounts due to the lessor for additional services and cost reimbursements.

In July 2019, the Company entered into a lease with Kadans Science Partner XII B.V. (“Kadans”), pursuant to which the Company agreed to lease approximately 5,070 square meters of office and laboratory space in a new multi-tenant office building that is to be constructed in Utrecht, the Netherlands. The initial term of the lease is ten years from the date that the premises are completed in accordance with certain specifications provided in a development agreement (described below), which is expected to occur in mid-2022. The lease will renew for two 5-year terms following the initial term, unless earlier terminated by the Company or Kadans, except that the earliest Kadans may terminate the lease is 20 years from the completion date. The lease provides for an estimated initial rent of approximately €1.3 million per annum. The rent amount is subject to adjustment based on the consumer price index (the “CPI”) beginning on January 1, 2019 through the completion date and then annually thereafter, subject to certain limitations if the CPI is greater than 3.0%. The final initial rent amount is contingent upon, among other things, the parameters of the final constructed premises, the final floor area, and the CPI adjustment described above, and will be determined upon the completion date and recorded in a first rider, signed by the Company and Kadans, to the lease. The Company is also responsible for certain fit-out costs and service fees related to the premises.

In July 2019, the Company also entered into a development agreement with Kadans and another party, Genmab B.V., which provides for the design, development and construction of the new multi-tenant office building of which the premises is a part.

The components of lease cost recorded in the Company’s consolidated statement of operations and statement of cash flows were as follows:

	For the Year Ended December 31,
	2021		2020
	(In thousands)
Operating lease cost	$	1,693	$	1,611
Variable lease cost		408		375
Total lease cost included in operating expenses	$	2,101	$	1,986

Cash paid to lessors included in operating cash outflows	$	1,704	$	2,478

The Company’s non-lease cost and other costs paid to the lessor are primarily related to services provided by the lessor in operating the premises that includes fees, operating costs, taxes and insurance related to the leased premises.

Maturities of the Company’s operating lease obligations as of December 31, 2021 were as follows (in thousands):

Year	Operating Leases
2022	$	1,616
2023		880
2024		645
2025		662
2026		222
Total lease payments		4,025
Less: amount representing interest		(274	)
Total lease obligations	$	3,751

The weighted-average remaining lease terms and discount rates related to the Company’s leases were as follows:

	As of December 31,
	2021			2020
Weighted-average remaining operating lease term (in years)		3.3			4.2
Weighted-average discount rate for operating leases		4.0	%		4.9	%

Investments in Debt Securities - Summary of Fair Value of Debt Securities by Major Security Type (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Total	$ 194,971	$ 62,327
Unrealized Gains	0	3
Unrealized Losses	(198)	(2)
Fair Value	194,773	62,328
Money market funds [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	5,684	10,156
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	5,684	10,156
Corporate paper and notes [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	155,039	27,978
Unrealized Gains	0	2
Unrealized Losses	(160)	0
Fair Value	154,879	27,980
U.S. government agency securities [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	2,093	9,150
Unrealized Gains	0	0
Unrealized Losses	(7)	0
Fair Value	2,086	9,150
U.S. treasuries [member]
Schedule Of Held To Maturity Securities [Line Items]
Total	32,155	15,043
Unrealized Gains	0	1
Unrealized Losses	(31)	(2)
Fair Value	$ 32,124	$ 15,042

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2021

Income Tax Disclosure [Abstract]

Summary of Components of Loss from Operations Before Income Tax Expense

The components of loss from operations before income tax expense are as follows:

	Year ended December 31,
	2021			2020			2019
				(In thousands)
United States	$	(8,502	)	$	(3,832	)	$	(1,363	)
Netherlands		(58,075	)		(81,178	)		(53,594	)
Total loss before income taxes	$	(66,577	)	$	(85,010	)	$	(54,957	)

Summary of Components of Expense for (Benefit) from Income Taxes

The components of income tax expense (benefit) from continuing operations are as follows:

	December 31,
	2021			2020			2019
	(In thousands)
U.S. federal	$	173		$	391		$	243
U.S. state		73			234			40
Total current tax expense	$	246		$	625		$	283

U.S. federal	$	(5	)	$	(86	)	$	(63	)
U.S. state		(2	)		(36	)		(26	)
Total deferred tax benefit	$	(7	)	$	(122	)	$	(89	)

Total income tax expense	$	239		$	503		$	194

Reconciliation of Netherlands Statutory Income Tax Rate and Effective Income Tax Rate

	Year Ended December 31,
	2021			2020			2019
Netherlands statutory income tax rate		25.0	%		25.0	%		25.0	%
Changes in tax rates		5.2			10.6			1.5
Non-deductible expenses		0.7			(2.4	)		(3.7	)
Change in valuation allowance		(31.4	)		(33.5	)		(23.3	)
Other		0.1			(0.3	)		0.1
Effective income tax rate		(0.4	)%		(0.6	)%		(0.4	)%

Summary of Components of Deferred Tax Assets (Liabilities)

The components of the Company’s deferred tax assets (liabilities) consist of the following:

	December 31,
	2021			2020
	(In thousands)
Deferred tax assets:
Net operating loss carryforwards	$	78,709		$	66,572
Deferred revenue		26,033			24,966
Excess interest carryforward		1,689			2,080
Lease obligation		1,006			1,058
Accrued expenses and other		478			494
Total deferred tax assets		107,915			95,170
Deferred tax asset valuation allowance		(106,440	)		(93,645	)
Total deferred tax assets, net of valuation allowance		1,475			1,525

Deferred tax liabilities:
Operating lease right-of-use assets	$	1,002		$	1,048
Other		56			67
Total deferred tax liabilities		1,058			1,115

Net deferred tax asset	$	417		$	410

Collaborations - Summary of Company's Accompanying Consolidated Financial Statements Attributable to Transactions Arising From Collaboration Arrangements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	$ 18,864	$ 18,193	$ 17,839
Lilly
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	0	0	0
Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	602	938	1,141
Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in the period	602	938	$ 1,141
Accounts Receivable \| Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	0
Billings	8,053
Cash receipts	(6,419)
Adjustments	0
Foreign exchange	0
Balance at December 31, 2021	1,634	0
Accounts Receivable \| Lilly
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	0
Billings	62,061
Cash receipts	(62,061)
Adjustments	0
Foreign exchange	0
Balance at December 31, 2021	0	0
Accounts Receivable \| Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	46
Billings	939
Cash receipts	(924)
Adjustments	0
Foreign exchange	(20)
Balance at December 31, 2021	41	46
Accounts Receivable \| Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	46
Billings	63,000
Cash receipts	(62,985)
Adjustments	0
Foreign exchange	(20)
Balance at December 31, 2021	41	46
Unbilled Receivables \| Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	1,623
Accrued receivables	9,361
Billings	(8,030)
Adjustments	21
Foreign exchange	0
Balance at December 31, 2021	2,975	1,623
Unbilled Receivables \| Lilly
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	0
Accrued receivables	3,264
Billings	(2,061)
Adjustments	0
Foreign exchange	0
Balance at December 31, 2021	1,203	0
Unbilled Receivables \| Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	0
Accrued receivables	1,408
Billings	(939)
Adjustments	0
Foreign exchange	(16)
Balance at December 31, 2021	453	0
Unbilled Receivables \| Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Balance at January 1, 2021	0
Accrued receivables	4,672
Billings	(3,000)
Adjustments	0
Foreign exchange	(16)
Balance at December 31, 2021	1,656	0
Deferred Revenue \| Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Allocation of contract consideration	0
Foreign exchange	(6,810)
Balance at January 1, 2021	99,004
Revenue recognized in the period	(18,864)
Balance at December 31, 2021	73,330	99,004
Less: current portion	(18,048)
Non-current balance at December 31, 2021	55,282
Deferred Revenue \| Lilly
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Allocation of contract consideration	43,523
Foreign exchange	(2,158)
Balance at January 1, 2021	0
Revenue recognized in the period	(14,012)
Balance at December 31, 2021	27,353	0
Less: current portion	(16,391)
Non-current balance at December 31, 2021	10,962
Deferred Revenue \| Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Allocation of contract consideration	0
Foreign exchange	(38)
Balance at January 1, 2021	862
Revenue recognized in the period	(602)
Balance at December 31, 2021	222	862
Less: current portion	(222)
Non-current balance at December 31, 2021	0
Deferred Revenue \| Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Allocation of contract consideration	43,523
Foreign exchange	(2,196)
Balance at January 1, 2021	862
Revenue recognized in the period	(14,614)
Balance at December 31, 2021	27,575	$ 862
Less: current portion	(16,613)
Non-current balance at December 31, 2021	$ 10,962

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) Segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounting Policies [Abstract]
Cash and cash equivalents, maturities	three months or less
Impairment charges of long-lived assets	$ 0	$ 0	$ 0
Reduction in research and development expenses	$ 9,300,000	$ 6,000,000.0	$ 4,500,000
Number of reportable segment \| Segment	1

Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Severance payment			$ 0.1
Additional compensation cost	$ 0.2
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net reversal of stock-based compensation expense			0.4
Other Losses, Net
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net loss associated with the derivative instrument		$ 1.0
401 (K) Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Defined contribution plan , description		The 401(k) Plan covers substantially all U.S. employees, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company matches contributions to the 401(k) Plan, matching 50% of an employee’s contribution up to a maximum of 3% of the participant’s compensation.
Percentage of employer matching contribution		50.00%
Defined contribution plan, cost		$ 0.1	0.1	$ 0.1
401 (K) Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Employer matching contribution, percent of employees compensation		3.00%
Postemployment Retirement Benefits
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Contributions to post-employment benefit plan		$ 2.9	$ 2.0	1.3
Executive settlement, cash compensation to be paid				0.9
Executive settlement expense				$ 1.8
Stock Option Activity
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation		$ 11.9
Weighted average remaining vesting period		1 year 3 months 18 days
Expected dividend yield		0.00%	0.00%	0.00%
Share-Based Payments
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield		0.00%
Share-Based Payments \| 2010 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments		4 years
Options expiration period		8 years
Share-Based Payments \| 2010 Plan \| First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage		25.00%
Share-Based Payments \| 2010 Plan \| Thereafter
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage		75.00%
Award vesting rights, required service period		36 months	36 months
Share-Based Payments \| 2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments		4 years	4 years
Options expiration period		10 years	10 years
Share-Based Payments \| 2016 Plan \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common shares authorized for issuance for future grants					2,836,603
Share-Based Payments \| 2016 Plan \| Non-Executive Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments		3 years	3 years
Share-Based Payments \| 2016 Plan \| Non-Executive Directors \| Subsequent Award
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options granted share-based payments		1 year	1 year
Award vesting rights, required service period		12 months	12 months
Share-Based Payments \| 2016 Plan \| First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage		25.00%	25.00%
Share-Based Payments \| 2016 Plan \| First Anniversary \| Non-Executive Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage		33.00%	33.00%
Share-Based Payments \| 2016 Plan \| Thereafter
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage		75.00%	75.00%
Award vesting rights, required service period		36 months	36 months
Share-Based Payments \| 2016 Plan \| Thereafter \| Non-Executive Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting rights, percentage		67.00%	67.00%
Award vesting rights, required service period		24 months	24 months
RSUs \| Postemployment Retirement Benefits
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Grant of RSUs				30,000
RSUs \| 2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Description of RSUs granted to employees		Each RSU represents the right to receive one common share.
Vesting period of RSUs granted share-based payments		4 years	4 years

Property and Equipment, net

12 Months Ended

Dec. 31, 2021

Property Plant And Equipment [Abstract]

Property and Equipment, net

5. Property and Equipment, net

Property and equipment, net consists of the following:

	December 31,
	2021			2020
	(In thousands)
Laboratory equipment	$	5,583		$	5,695
Office equipment and furniture		1,222			1,300
Leasehold improvements		111			117
Construction in progress		1,056			496
Property and equipment		7,972			7,608
Less: accumulated depreciation and amortization		(4,423	)		(3,493	)
Property and equipment, net	$	3,549		$	4,115

Construction in progress relates to certain development and construction costs related to the office lease the Company entered into with Kadans Science Partner XII B.V. that is expected to complete during 2022. Additional details for the lease agreement is described in Note 9 Operating Leases. Depreciation and amortization expense was $1.2 million, $1.2 million and $0.9 million for the years ended December 31, 2021, 2020, and 2019, respectively. Property and equipment are predominantly located in the Netherlands.

Loss per Share - Summary of Basic and Diluted Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net loss	$ (66,816)	$ (85,513)	$ (55,151)
Weighted average shares outstanding	38,638,434	29,256,203	24,218,083
Basic and diluted	$ (1.73)	$ (2.92)	$ (2.28)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8933-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Collaborations (Tables)

12 Months Ended

Dec. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Summary of Company's Accompanying Consolidated Financial Statements Attributable to Transactions Arising From Collaboration Arrangements

	Related Party			Third Party
	Incyte			Lilly			Other			Total
Contract assets
Accounts receivable
Balance at January 1, 2021	$	—		$	—		$	46		$	46
Billings		8,053			62,061			939			63,000
Cash receipts		(6,419	)		(62,061	)		(924	)		(62,985	)
Adjustments		—			—						—
Foreign exchange		—			—			(20	)		(20	)
Balance at December 31, 2021		1,634			—			41			41

Unbilled receivables
Balance at January 1, 2021	$	1,623		$	—		$	—		$	—
Accrued receivables		9,361			3,264			1,408			4,672
Billings		(8,030	)		(2,061	)		(939	)		(3,000	)
Adjustments		21			—			—			—
Foreign exchange		—			—			(16	)		(16	)
Balance at December 31, 2021		2,975			1,203			453			1,656

Contract liabilities
Deferred revenue
Balance at January 1, 2021	$	99,004		$	—		$	862		$	862
Allocation of contract consideration		—			43,523			—			43,523
Revenue recognized in the period		(18,864	)		(14,012	)		(602	)		(14,614	)
Foreign exchange		(6,810	)		(2,158	)		(38	)		(2,196	)
Balance at December 31, 2021		73,330			27,353			222			27,575
Less: current portion		(18,048	)		(16,391	)		(222	)		(16,613	)
Non-current balance at December 31, 2021		55,282			10,962			—			10,962

Summary of Company's Collaboration Arrangements Earned in Period to Be Billed and Collected in Next Period

	For the Year Ended December 31, 2021
	Related Party		Third Party
	Incyte		Lilly		Other			Total
Upfront payments	$	18,864	$	14,012	$	602		$	14,614
Reimbursement revenue		8,740		3,332		1,057			4,389
Milestones		2,000		—		500			500
Total collaboration revenue	$	29,604	$	17,344	$	2,159		$	19,503

Operating expenses:
Research and development expense	$	1,223	$	—	$	151		$	151
General and administrative expense		—		—		—			—
Total operating expenses from collaborations	$	1,223	$	—	$	151		$	151

Revenue recognized that was included in deferred revenue at the beginning of the period	$	18,864	$	—	$	602		$	602

	For the Year Ended December 31, 2020
	Related Party		Third Party
	Incyte		Lilly		Other			Total
Upfront payments	$	18,193	$	—	$	895		$	895
Reimbursement revenue		8,387		—		(12	)		(12	)
Milestones		—		—		2,480			2,480
Total collaboration revenue	$	26,580	$	—	$	3,363		$	3,363

Operating expenses:
Research and development expense	$	2,036	$	—	$	1,944		$	1,944
General and administrative expense		—		—		—			—
Total operating expenses from collaborations	$	2,036	$	—	$	1,944		$	1,944

Revenue recognized that was included in deferred revenue at the beginning of the period	$	18,193	$	—	$	938		$	938

	For the Year Ended December 31, 2019
	Related Party		Third Party
	Incyte		Lilly		Other			Total
Upfront payments	$	17,839	$	—	$	839		$	839
Reimbursement revenue		7,992		—		252			252
Milestones		—		—		4,426			4,426
Total collaboration revenue	$	25,831	$	—	$	5,517		$	5,517

Operating expenses:
Research and development expense	$	680	$	—	$	—		$	—
General and administrative expense		—		—		—			—
Total operating expenses from collaborations	$	680	$	—	$	—		$	—

Revenue recognized that was included in deferred revenue at the beginning of the period	$	17,839	$	—	$	1,141		$	1,141

Intangible Assets, Net - Summary of Intangible Assets, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 3,863	$ 4,186
Less: accumulated amortization	(1,516)	(1,343)
Intangible assets, net	2,347	2,843
Licenses of Intellectual Property [Member]
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	3,597	3,898
Software Licenses [Member]
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 266	$ 288

Income Taxes - Summary of Components of Expense for (Benefit) from Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
U.S. federal	$ 173	$ 391	$ 243
U.S. state	73	234	40
Total current tax expense	246	625	283
U.S. federal	(5)	(86)	(63)
U.S. state	(2)	(36)	(26)
Total deferred tax benefit	(7)	(122)	(89)
Total income tax expense	$ 239	$ 503	$ 194

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

Operating Leases - Schedule of Maturities of Operating Lease Obligations (Details) $ in Thousands	Dec. 31, 2021 USD ($)
Lease Cost [Abstract]
2022	$ 1,616
2023	880
2024	645
2025	662
2026	222
Total lease payments	4,025
Less: amount representing interest	(274)
Total lease obligations	$ 3,751

Collaborations - Summary of Company's Collaboration Arrangements Earned in Period to Be Billed and Collected in Next Period (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ 98,187	$ 70,040	$ 55,680
General and administrative	40,896	35,781	34,110
Total operating expenses	139,083	105,821	89,790
Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	18,864	18,193	17,839
Reimbursement revenue	8,740	8,387	7,992
Milestones	2,000	0	0
Total collaboration revenue	29,604	26,580	25,831
Operating expenses:
Research and development	1,223	2,036	680
General and administrative	0	0	0
Total operating expenses	1,223	2,036	680
Revenue recognized in the period	18,864	18,193	17,839
Lilly
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	14,012	0	0
Reimbursement revenue	3,332	0	0
Milestones	0	0	0
Total collaboration revenue	17,344	0	0
Operating expenses:
Research and development	0	0	0
General and administrative	0	0	0
Total operating expenses	0	0	0
Revenue recognized in the period	0	0	0
Other
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	602	895	839
Reimbursement revenue	1,057	(12)	252
Milestones	500	2,480	4,426
Total collaboration revenue	2,159	3,363	5,517
Operating expenses:
Research and development	151	1,944	0
General and administrative	0	0	0
Total operating expenses	151	1,944	0
Revenue recognized in the period	602	938	1,141
Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payments	14,614	895	839
Reimbursement revenue	4,389	(12)	252
Milestones	500	2,480	4,426
Total collaboration revenue	19,503	3,363	5,517
Operating expenses:
Research and development	151	1,944	0
General and administrative	0	0	0
Total operating expenses	151	1,944	0
Revenue recognized in the period	$ 602	$ 938	$ 1,141

Operating Leases - Schedule of Weighted Average Remaining Lease Terms and Discount Rates (Details)	Dec. 31, 2021	Dec. 31, 2020
Leases [Abstract]
Weighted-average remaining operating lease term (in years)	3 years 3 months 18 days	4 years 2 months 12 days
Weighted-average discount rate for operating leases	4.00%	4.90%

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Preparation

Principles of Consolidation

Functional and Presentation Currency

Use of Estimates

Concentration of Credit Risk

Subsequent Events

Fair Value Measurements

•

Level 1 – Quoted market prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

•

The asset’s or liability’s fair value measurement within the fair value hierarchy is based upon the lowest level of any input that is significant to the fair value measurement.

Going Concern

Cash and Cash Equivalents

Restricted Cash

Marketable Securities

Accounts Receivable

Accounts receivable are amounts due from collaboration partners as a result of research and development services provided or milestones achieved but not yet paid.

Allowance for Credit Losses

Property and Equipment

The Company records property and equipment at cost. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:

Asset Category		Useful Lives
Laboratory equipment		5 years
Office furniture and equipment		5 years
Leasehold improvements		Shorter of useful life or term of lease

Intangible Assets

Impairment of Long-Lived Assets

Leases

Income Taxes

Deferred Taxes

Unrecognized Tax Benefits

Revenue Recognition

identify the contract(s) with a customer;

ii.

identify the performance obligations in the contract;

iii.

determine the transaction price;

iv.

allocate the transaction price to the performance obligations in the contract; and

recognize revenue when (or as) the entity satisfies a performance obligation.

performed on behalf of the collaboration partner related to the licensed targets. The Company also derives revenue from government grants.

As part of the accounting for these arrangements, the Company must use judgment to determine:

the number of performance obligations based on the determination under step (ii) above and whether those performance obligations are distinct from other performance obligations in the contract;

the transaction price under step (iii) above;

the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above;

whether the combined performance obligation is satisfied over time or at a point in time in step (v) above; and

the appropriate method for measuring progress toward complete satisfaction of a performance obligation in step (v) above.

Exclusive Licenses

Research and Development Services

Government Grants

Research and Development Expenses

Share-Based Payments

Earnings (Loss) per Share

Segment Information

The Company operates in one reportable segment, which comprises the discovery and development of innovative therapeutics.

Recently Adopted Accounting Pronouncements

Employee Benefit Plans - Summary of RSU Activity (Details) - RSUs	12 Months Ended
	Dec. 31, 2021 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of RSUs, Non-vested, Beginning balance \| shares	59,775
Number of RSUs, Granted \| shares	24,976
Number of RSUs, Vested \| shares	(38,882)
Number of RSUs, Forfeited \| shares	(35,869)
Number of RSUs, Non-vested, Ending balance \| shares	10,000
Weighted Average Grant-date Fair Value, Non-vested, Beginning balance \| $ / shares	$ 16.48
Weighted Average Grant-date Fair Value, Granted \| $ / shares	23.40
Weighted Average Grant-date Fair Value, Vested \| $ / shares	17.17
Weighted Average Grant-date Fair Value, Forfeited \| $ / shares	17.15
Weighted Average Grant-date Fair Value, Non-vested, Ending balance \| $ / shares	$ 25.56

Commitments and Contingencies

12 Months Ended

Dec. 31, 2021

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Indemnities

The Company indemnifies its officers and directors for certain events or occurrences while the officer or director is, or was, serving at the Company’s request in such capacity. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ and officers’ insurance coverage that is intended to limit its exposure and enable it to recover a portion of any future amounts paid.

The Company enters into certain agreements with other parties in the ordinary course of business that contain indemnification provisions. These typically include agreements with directors and officers, business partners, contractors, landlords, clinical sites and customers. Under these provisions, the Company may indemnify and hold harmless the indemnified party for losses suffered or incurred by the indemnified party as a result of the Company’s activities, such as gross negligence, willful misconduct or at times, other activities. These indemnification provisions may survive termination of the underlying agreements. The maximum potential amount of future payments the Company could be required to make under these indemnification provisions may be unlimited. However, to date the Company has not incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. As a result, the estimated fair value of these obligations is minimal. Accordingly, the Company did not have any liabilities recorded for these obligations as of December 31, 2021.

Litigation

On April 5, 2018, an unnamed third party filed a notice of opposition against the Company’s EP 2604625 patent, entitled “Generation of Binding Molecules,” in the European Opposition Division of the European Patent Office (the “EPO”). The notice asserted, as applicable, added subject matter, lack of novelty, lack of inventive step, and insufficiency. On August 20, 2018, the Company timely responded to these submissions. An opposition hearing was held in June 2019, wherein the EPO revoked the EP 2604625 patent in its entirety under Art. 123(2) EPC. The Company timely appealed that decision in December 2019 before the Technical Board of Appeals for the EPO seeking reinstatement of the patent and proposing auxiliary requests for certain amended claims, with further proceedings scheduled in May 2022. As this opposition proceeding continues, the Company cannot be certain that it will ultimately prevail.

From time to time, the Company may be involved in various other claims and legal proceedings relating to claims arising out of the Company’s operations. The Company is not currently a party to any material legal proceedings.