Immunogen (IMGN) 10-K 1 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 18, 2021	Jun. 30, 2020
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	0-17999
Entity Registrant Name	ImmunoGen, Inc.
Entity Incorporation, State or Country Code	MA
Entity Tax Identification Number	04-2726691
Entity Address, Address Line One	830 Winter Street
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	781
Local Phone Number	895-0600
Title of 12(b) Security	Common Stock
Trading Symbol	IMGN
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		199,731,834
Entity Central Index Key	0000855654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Public Float			$ 799,032,429

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Cash and cash equivalents	$ 293,856	$ 176,225
Accounts receivable	35	7,500
Unbilled revenue	11	1,001
Contract assets		3,631
Non-cash royalty receivable	22,451	15,116
Prepaid and other current assets	7,901	5,425
Total current assets	324,254	208,898
Property and equipment, net of accumulated depreciation	5,760	6,993
Operating lease right-of-use assets	14,072	15,587
Other assets	10,986	3,784
Total assets	355,072	235,262
LIABILITIES AND SHAREHOLDERS' EQUITY (DEFICIT)
Accounts payable	9,538	9,933
Accrued compensation	4,620	8,991
Other accrued liabilities	29,320	13,932
Convertible 4.5% senior notes, net of deferred financing costs of $7	2,093
Current portion of liability related to the sale of future royalties, net of deferred financing costs of $319 and $635, respectively	44,357	41,274
Current portion of operating lease liability	3,146	2,971
Current portion of deferred revenue	29,249	309
Total current liabilities	122,323	77,410
Deferred revenue, net of current portion	80,860	127,123
Operating lease liability, net of current portion	18,651	21,798
Convertible 4.5% senior notes, net of deferred financing costs of $22		2,078
Liability related to the sale of future royalties, net of current portion and deferred financing costs of $584 and $859, respectively	41,082	82,267
Other long-term liabilities	2,586	707
Total liabilities	265,502	311,383
Commitments and contingencies (Note K)
Shareholders deficit:
Preferred stock, $.01 par value; authorized 5,000 shares; no shares issued and outstanding as of December 31, 2020 and December 31, 2019, respectively
Common stock, $.01 par value; authorized 300,000 and 200,000 shares; issued and outstanding 194,998 and 150,136 shares as of December 31, 2020 and December 31, 2019, respectively	1,950	1,501
Additional paid-in capital	1,419,460	1,209,846
Accumulated deficit	(1,331,840)	(1,287,468)
Total shareholders' equity (deficit)	89,570	(76,121)
Total liabilities and shareholders' equity (deficit)	$ 355,072	$ 235,262

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
CONSOLIDATED BALANCE SHEETS
Current portion of deferred financing costs	$ 7
Interest rate (as a percent)	4.50%	4.50%
Non-current deferred financing costs	$ 22
Sale of future royalties, current portion and deferred financing costs	319	$ 635
Sale of future royalties, noncurrent portion and deferred financing costs	$ 584	$ 859
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, authorized shares	5,000	5,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized shares	300,000	200,000
Common stock, issued shares	194,998	150,136
Common stock, outstanding shares	194,998	150,136

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Total revenues	$ 132,299	$ 82,271	$ 53,446
Operating expenses:
Research and development	114,592	114,522	174,456
General and administrative	38,600	38,489	36,746
Restructuring charge	1,487	21,433	3,693
Total operating expenses	154,679	174,444	214,895
Loss from operations	(22,380)	(92,173)	(161,449)
Investment income, net	729	4,424	4,227
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(23,107)	(16,879)	(10,631)
Interest expense on convertible senior notes	(95)	(95)	(95)
Other income (expense), net	481	590	(895)
Net loss	$ (44,372)	$ (104,133)	$ (168,843)
Basic and diluted net loss per common share	$ (0.25)	$ (0.70)	$ (1.21)
Basic and diluted weighted average common shares outstanding	176,153	148,311	139,946
Total comprehensive loss	$ (44,372)	$ (104,133)	$ (168,843)
License and milestone fees
Revenues:
Total revenues	63,742	34,788	15,280
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	68,529	47,415	32,154
Research and development support
Revenues:
Total revenues	$ 28	$ 68	1,377
Clinical materials revenue
Revenues:
Total revenues			$ 4,635

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated DeficitAdjustment	Accumulated Deficit	Adjustment	Total
Increase (Decrease) in Shareholders' Equity (Deficit)
Transition adjustment for ASC 606			$ 14,090		$ 14,090
Balance at Dec. 31, 2017	$ 1,325	$ 1,009,362		$ (1,028,582)		$ (17,895)
Balance (in shares) at Dec. 31, 2017	132,526
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(168,843)		(168,843)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 9	4,292				4,301
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	946
Issuance of common stock, net of issuance costs	$ 158	162,354				162,512
Issuance of common stock, net of issuance costs (in shares)	15,755
Stock option and restricted stock compensation expense		16,445				16,445
Directors' deferred share units converted	$ 2	(1)				1
Directors' deferred share units converted (in shares)	173
Directors' deferred share unit compensation		361				361
Balance at Dec. 31, 2018	$ 1,494	1,192,813		(1,183,335)		10,972
Balance (in shares) at Dec. 31, 2018	149,400
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(104,133)		(104,133)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 11	2,862				2,873
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	1,150
Stock options exercised		13,830				13,830
Restricted stock award, net of forfeitures	$ (4)	4
Restricted stock award, net of forfeitures (in shares)	(414)
Directors' deferred share unit compensation		337				337
Balance at Dec. 31, 2019	$ 1,501	1,209,846		(1,287,468)		$ (76,121)
Balance (in shares) at Dec. 31, 2019	150,136					150,136
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss				(44,372)		$ (44,372)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 5	1,466				1,471
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	458
Issuance of common stock, net of issuance costs	$ 445	193,826				194,271
Issuance of common stock, net of issuance costs (in shares)	44,496
Restricted stock units vested	$ 4	(4)
Restricted stock units vested (in shares)	395
Restricted stock award forfeitures	$ (5)	5
Restricted stock award forfeitures (in shares)	(487)
Stock option and restricted stock compensation expense		13,978				13,978
Directors' deferred share unit compensation		343				343
Balance at Dec. 31, 2020	$ 1,950	$ 1,419,460		$ (1,331,840)		$ 89,570
Balance (in shares) at Dec. 31, 2020	194,998					194,998

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (44,372)	$ (104,133)	$ (168,843)
Adjustments to reconcile net loss to net cash used for operating activities:
Non-cash royalty revenue related to sale of future royalties	(68,529)	(47,415)	(32,154)
Non-cash interest expense on liability related to sale of future royalties and convertible senior notes	23,107	16,879	10,631
Depreciation and amortization	2,101	4,028	7,411
(Gain) loss on sale/disposal of fixed assets and impairment charges	(691)	1,689	115
Operating lease right-of-use asset impairment		694
Stock and deferred share unit compensation	14,321	14,167	16,807
Deferred rent			(95)
Change in operating assets and liabilities:
Accounts receivable	7,465	(5,799)	948
Unbilled receivable	990	(384)	1,963
Inventory			1,038
Contract asset	3,631	(3,131)	(500)
Prepaid and other current assets	(2,476)	(963)	(1,495)
Operating lease right-of-use assets	1,515	1,331
Other assets	(7,202)	(75)	88
Accounts payable	(819)	(1,045)	2,667
Accrued compensation	(4,100)	(2,189)	323
Other accrued liabilities	16,734	(6,146)	3,839
Deferred revenue	(17,323)	46,630	(9,165)
Operating lease liability	(2,972)	(2,505)
Net cash used for operating activities	(78,620)	(88,367)	(166,422)
Cash flows from investing activities:
Purchases of property and equipment	(917)	(2,845)	(5,246)
Proceeds from sale of equipment	1,426	2,312
Net cash provided by (used for) investing activities	509	(533)	(5,246)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock plans	1,471	2,873	4,301
Proceeds from common stock issuance, net of $701 and $395 of transaction costs, respectively	194,271		162,512
Net cash provided by financing activities	195,742	2,873	166,813
Net change in cash and cash equivalents	117,631	(86,027)	(4,855)
Cash and cash equivalents, beginning of period	176,225	262,252	267,107
Cash and cash equivalents, end of period	293,856	176,225	262,252
Supplemental cash flow information:
Cash paid during the year for interest	$ 95	$ 95	$ 95

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
CONSOLIDATED STATEMENTS OF CASH FLOWS
Transaction costs	$ 701	$ 395

Nature of Business and Plan of

Nature of Business and Plan of Operations	12 Months Ended
	Dec. 31, 2020
Nature of Business and Plan of Operations
Nature of Business and Plan of Operations	A. Nature of Business and Plan of Operations ImmunoGen, Inc. (the Company) was incorporated in Massachusetts in 1981 and is focused on the development of antibody-drug conjugates, or ADCs. The Company has generally incurred operating losses and negative cash flows from operations since inception, incurred a net loss of $44.4 million during the year ended December 31, 2020, and has an accumulated deficit of approximately $1.3 billion as of December 31, 2020. The Company has primarily funded these losses through payments received from its collaborations and equity, convertible debt, and other financings. To date, the Company has no product revenue and management expects to continue to incur operating expenses related to research and development and potential commercialization of its portfolio over the next several years. At December 31, 2020, the Company had $293.9 million of cash and cash equivalents on hand. The Company anticipates that its current capital resources, inclusive of $33.6 million of net proceeds generated from an Open Market Sale Agreement SM The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, the development by its competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, manufacturing and marketing limitations, complexities associated with managing collaboration arrangements, third-party reimbursements, and compliance with governmental regulations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	B. Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, ImmunoGen Securities Corp., ImmunoGen Europe Limited, ImmunoGen BioPharma (Ireland) Limited, and Hurricane, LLC. All intercompany transactions and balances have been eliminated. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States (U.S.) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Subsequent Events The Company has evaluated all events or transactions that occurred after December 31, 2020 up through the date the Company issued these financial statements. Pursuant to an Open Market Sale Agreement SM under which the Company may issue and sell shares of its common stock, from time to time for an aggregate sales price of up to $150.0 million, subsequent to December 31, 2020 and through the date the Company issued these financial statements, the Company has sold 4,544,424 shares of its common stock generating net proceeds of $33.6 million after deducting offering commissions and expenses. The Company did not have any other material recognized or unrecognized subsequent events. Adoption of ASC 842, Leases The Company adopted Accounting Standards Update (ASU) No. 2016-2, Leases (Topic 842) Leases (Topic 842): Targeted Improvements the previous lease guidance of ASC 840, Leases Revenue Recognition The Company enters into licensing and development agreements with collaborators for the development of ADCs. The terms of these agreements contain multiple promised goods and services which may include (i) licenses, or options to obtain licenses, to the Company’s ADC technology, (ii) rights to future technological improvements, (iii) technology transfer services and other activities to be performed on behalf of the collaborative partner, and (iv) delivery of cytotoxic agents and/or the manufacture of preclinical and clinical materials for the collaborative partner. Payments to the Company under these agreements may include upfront fees, option fees, exercise fees, payments for services, payments for preclinical or clinical materials, payments based upon the achievement of certain milestones, and royalties on product sales. The Company follows the provisions of ASC 606, Revenue from Contracts with Customers, Revenue is recognized when a 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. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when or as the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations based on its assessment of whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when or as the performance obligation is satisfied. The Company exercises judgment in assessing those promised goods and services that are distinct and thus representative of performance obligations. To the extent the Company identifies multiple performance obligations in a contract, the Company must develop assumptions that require judgment to determine the estimated standalone selling price for each performance obligation in order to allocate the transaction price among the identified performance obligations. These judgments and assumptions are discussed in further detail below. At December 31, 2020, the Company had the following types of material agreements with the parties identified below: ● Development and commercialization licenses, which provide the counterparty with the right to use the Company’s ADC technology and/or certain other intellectual property to develop and commercialize compounds to a specified antigen target: Bayer (one exclusive single-target license) CytomX (two exclusive single-target licenses) Debiopharm (one exclusive single-compound license) Fusion Pharmaceuticals (one exclusive single-target license) Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (one territory-specific exclusive single-compound license) Novartis (five exclusive single-target licenses) Oxford BioTherapeutics/Menarini (one exclusive single-target license sublicensed from Amgen) Roche, through its Genentech unit (five exclusive single-target licenses) Viridian (one exclusive single-target license) ● Collaboration and license agreement to co-develop and co-commercialize a specified anticancer compound on established terms: MacroGenics During the year ended December 31, 2020, pursuant to notices received, the exclusive development and commercialization licenses granted to each of Biotest and Takeda and the collaboration and option agreement with Jazz were terminated. There are no performance, cancellation, termination, or refund provisions in any of the arrangements that contain material financial consequences to the Company. Development and Commercialization Licenses The obligations under a development and commercialization license agreement generally include the license to the Company’s ADC technology with respect to a specified antigen target or compound, and may also include obligations related to rights to future technological improvements and other activities to be performed on behalf of the collaborative partner. Generally, development and commercialization licenses contain non-refundable terms for payments and, depending on the terms of the agreement, provide that the Company will earn payments upon the achievement of certain milestones and royalty payments, generally until the later of the last applicable patent expiration or a fixed period of years after product launch. Royalty rates may vary over the royalty term depending on the Company’s intellectual property rights and/or the presence of comparable competing products. In the case of Debiopharm, no royalties will be received. In certain instances, the Company may also provide cytotoxic agents and/or clinical materials or other services in addition to the development and commercialization licenses. For example, the Company may provide technology transfer services in connection with the out-licensing of product candidates initially developed by the Company, and may also provide technical assistance and share any technology improvements with its collaborators during the term of the collaboration agreements. The Company does not directly control when or whether any collaborator will request certain services, achieve milestones, or become liable for royalty payments. In determining the performance obligations for these arrangements, management evaluates whether the license is distinct and has significant standalone functionality either alone or with other readily available resources based on the consideration of the relevant facts and circumstances for each arrangement. Factors considered in this determination include the research capabilities of the partner and the availability of ADC technology research expertise and ADC manufacturing capabilities in the general marketplace and whether technological improvements are required for the continued functionality of the license. If the license to the Company’s intellectual property is determined to be distinct, the Company recognizes revenues 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. If the license is not distinct, the license is combined with other goods or services into a single performance obligation and revenue is recognized over time. The Company estimates the standalone selling prices of the license and all other performance obligations based on market conditions, similar arrangements entered into by third parties, and entity-specific factors such as the terms of the Company’s previous collaborative agreements, recent preclinical and clinical testing results of therapeutic products that use the Company’s ADC technology, the Company’s pricing practices and pricing objectives, the likelihood that technological improvements will be made, and, if made, will be used by the Company’s collaborators, and the nature of the other services to be performed on behalf of its collaborators and market rates for similar services. The Company recognizes revenue related to technology transfer activities and other services as the services are performed. The Company is generally compensated for these activities at negotiated rates that are consistent with what other third parties would charge. The Company records amounts recognized for research materials provided or services performed as a component of research and development support revenue. The Company may also provide cytotoxic agents and/or preclinical and clinical materials (drug substance/drug product) to its collaborators at negotiated prices generally consistent with what other third parties would charge. The Company recognizes revenue on cytotoxic agents and on preclinical and clinical materials when control transfers to the collaborator. The Company recognizes revenue related to the rights to future technological improvements over the estimated term of the applicable license. The Company’s development and commercialization license agreements have milestone payments which for reporting purposes are aggregated into two categories: (i) development and regulatory milestones, and (ii) sales milestones. Development milestones are typically payable when a product candidate initiates or advances into different clinical trial phases. Regulatory milestones are typically payable upon submission for marketing approval with the FDA or other countries’ regulatory authorities or on receipt of actual marketing approvals for the compound or for additional indications. Sales milestones are typically payable when annual sales reach certain levels. At the inception of each arrangement, the Company evaluates any development and regulatory milestone payments to determine whether the milestone is considered probable of being reached 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 milestone value is included in the transaction price to be allocated; otherwise, such amounts are considered constrained and excluded from the transaction price. As part of its evaluation of the constraint, the Company considers numerous factors, including whether the achievement of the milestone is outside the control of the Company and contingent upon the future success of clinical trials, the collaborator’s efforts, or the receipt of regulatory approval. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development or regulatory milestones and any related constraint, and if necessary, adjusts the estimate of the transaction price. In addition, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. Amounts allocated to a satisfied performance obligation are recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. For development and commercialization license agreements that include sales-based royalties, including milestone payments based on the level of sales, 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) in accordance with the royalty recognition constraint. Under the Company’s development and commercialization license agreements, except for the Debiopharm license, the Company receives royalty payments based upon its licensees’ net sales of covered products. Generally, under the development and commercialization agreements, the Company receives royalty reports and payments from its licensees approximately one quarter in arrears. The Company estimates the amount of royalty revenue to be recognized based on historical and forecasted sales and/or sales information from its licensees if available. Collaboration and Option Agreements/Right-to-Test Agreements The Company’s right-to-test agreements provide collaborators the right to test the Company’s ADC technology for a defined period of time through a research, or right-to-test, license. Under both right-to-test agreements and collaboration and option agreements, collaborators may (a) “take” options, for a defined period of time, to specified targets and (b) upon exercise of those options, secure or “take” licenses to develop and commercialize products for the specified targets on established terms. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement (referred to as “upfront” fees or payments), (ii) upon the opt-in to acquire a development and commercialization license(s) (referred to as exercise fees or payments earned, if any, when the development and commercialization license is “taken”), (iii) after providing services at the collaborator’s request at negotiated prices, which are generally consistent with what other third parties would charge, or (iv) upon some combination of all of these fees. The accounting for collaboration and option agreements and right-to-test agreements is dependent on the nature of the options granted to the collaborative partner. Options are considered distinct performance obligations if they provide a collaborator with a material right. Factors that are considered in evaluating whether options convey a material right include the overall objective of the arrangement, the benefit the collaborator might obtain from the agreement without exercising the options, the cost to exercise the options relative to the fair value of the licenses, and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. As of December 31, 2020, all option and right-to-test agreements have expired or terminated. If the Company concludes that an option provides the customer a material right, and therefore is a separate performance obligation, the Company then determines the estimated standalone selling price of the option using the following inputs: (a) estimated fair value of the license underlying each option, (b) the amount the partner would pay to exercise the option to obtain the license, and (c) probability of exercise. The Company does not control when or if any collaborator will exercise its options for development and commercialization licenses. As a result, the Company cannot predict when or if it will recognize revenues in connection with any of the foregoing. Upfront payments on development and commercialization licenses may be recognized upon delivery of the license if facts and circumstances dictate that the license is distinct from the other promised goods and services. In determining whether a collaboration and option agreement is within the scope of ASC 808, Collaborative Arrangements Transaction Price Allocated to Remaining Performance Obligations Deferred revenue under ASC 606 represents the portion of the transaction price received under various contracts for which work has not been performed (or has been partially performed) and includes unexercised contract options that are considered material rights. As of December 31, 2020, the aggregate amount of the transaction price allocated to remaining performance obligations comprising deferred revenue was $110.1 million. The Company expects to recognize revenue on approximately 27%, 65%, and 8% of the remaining performance obligations over the next 12 months , 13 to 60 months , and 61 to 120 months , respectively, however, it does not control when or if any collaborator will terminate existing development and commercialization licenses. Contract Balances from Contracts with Customers The following tables present changes in the Company’s contract assets and contract liabilities during the years ended December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Year ended December 31, 2020 ​ December 31, 2019 Additions ​ Deductions ​ Impact of Netting ​ December 31, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ 42,050 ​ $ (63,742) ​ $ 4,369 ​ $ 110,109 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Year ended December 31, 2019 ​ December 31, 2018 ​ Additions ​ Deductions ​ Impact of Netting ​ December 31, 2019 Contract asset ​ $ 500 ​ $ 8,000 ​ $ (500) ​ $ (4,369) ​ $ 3,631 Contract liabilities (deferred revenue) ​ $ 80,802 ​ $ 65,816 ​ $ (14,817) ​ $ (4,369) ​ $ 127,432 ​ During the years ended December 31, 2020, 2019, and 2018 the Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ ​ December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 61,872 ​ $ 14,817 ​ $ 14,139 ​ Performance obligations satisfied in previous periods ​ $ — ​ $ 12,672 ​ $ 1,476 ​ ​ During 2020, the Company recognized $60.5 million of previously deferred license revenue upon Jazz’s opt-out of its right to the last remaining license under the agreement and $3.2 million of upfront fees previously received from other partners, of which $1.4 million was included in contract liabilities at the beginning of 2020. A $40.0 million upfront payment received in 2020 pursuant to a license agreement executed with Huadong was recorded as deferred revenue and none of this amount was recognized as revenue during 2020. Additionally, a contract asset of $3.6 million, net of $4.4 million in related contract liabilities, was recorded for two probable milestones in 2019 pursuant to license agreements with CytomX and Novartis, which were subsequently achieved and paid during 2020. The Company recorded the following during the year ended December 31, 2019: (i) license and milestone fee revenue of $7.7 million for probable development milestones pursuant to license agreements with CytomX and Novartis, with another $0.3 million deferred which represents the amount allocated to future rights to technological improvements; a $3.6 million contract asset was recorded in December 2019 related to these probable milestones, net of a $4.4 million reduction in related contract liabilities; (ii) a $5 million regulatory milestone payment earned under its license agreement with Genentech, a member of the Roche Group; the full amount of the milestone was recognized as revenue in the period as the amount allocated to future rights to technological improvements was not material; (iii) $14.5 million of previously deferred license revenue recognized upon the opt-out of the right to execute a license by Jazz; (iv) $65.2 million was recorded as deferred revenue as a result of a sale of the Company’s residual rights to receive royalty payments on commercial sales of Kadcyla ® As a result of adoption of ASC 606, a contract asset of $5.0 million was recorded for a probable milestone which was subsequently earned and paid during the year ended December 31, 2018. Additionally the Company recorded the following during 2018: (i) a contract asset and related revenue of $0.5 million for a probable milestone pursuant to a license agreement with Fusion Pharmaceuticals, which was subsequently paid in 2019; (ii) a $1 million development milestone earned under a sublicense agreement with Oxford BioTherapeutics Ltd. as license and milestone fee revenue, which was included in accounts receivable as of December 31, 2018; (iii) $10.9 million of revenue previously deferred, with a net reduction in deferred revenue of $5.9 million due to contract asset and contract liability netting as a result of Takeda not executing a second license it had available, or extending or expanding its right-to-test agreement; (iv) $0.8 million of revenue previously deferred upon completion of Debiopharm and another collaborator’s performance obligations; (v) $2.1 million of revenue previously deferred related to numerous collaborators’ rights to technological improvements; and (vi) $0.3 million of revenue previously deferred upon shipment of clinical materials to a partner which is included in clinical material revenue. The timing of revenue recognition, billings, and cash collections results in billed receivables, unbilled receivables, contract assets, and contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded (under the caption deferred revenue). Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met. Financial Instruments and Concentration of Credit Risk Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. Deposits with banks may exceed the amount of insurance provided on such deposits. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal risk. The Company’s cash equivalents consist of money market funds with underlying investments primarily being U.S. Government-issued securities and high quality, short-term commercial paper. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents, and marketable securities. The Company held no marketable securities as of December 31, 2020 or 2019. The Company’s investment policy, approved by the Board of Directors, limits the amount it may invest in any one type of investment, thereby reducing credit risk concentrations. Cash and Cash Equivalents All highly liquid financial instruments with maturities of three months or less when purchased are considered cash equivalents. As of December 31, 2020 and 2019, the Company held $293.9 million and $176.2 million, respectively, in cash and money market funds consisting principally of U.S. Government-issued securities and high quality, short-term commercial paper which were classified as cash and cash equivalents. Non-cash Investing Activities The Company had $0.7 million of accrued capital expenditures as of December 31, 2020 which have been treated as a non-cash investing activity and, accordingly, not reflected in the consolidated statement of cash flows. The Company had no accrued capital expenditures as of December 31, 2019. Fair Value of Financial Instruments ASC 820, Fair Value Measurement ● Level 1—Quoted prices in active markets for identical assets or liabilities. ● Level 2—Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of December 31, 2020 and 2019, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following tables represent the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of each date (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 194,525 ​ $ 194,525 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 163,674 ​ $ 163,674 $ — $ — ​ The fair value of the Company’s cash equivalents is based primarily on quoted prices from active markets. The carrying amounts reflected in the consolidated balance sheets for accounts receivable, unbilled receivable, contract assets, non-cash royalty receivable, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. Unbilled Receivable Unbilled receivable primarily represents research funding earned based on actual resources utilized and external expenses incurred under certain of the Company’s collaborator agreements. Clinical Trial Accruals Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these activities to third parties. Third-party clinical trial expenses include investigator fees, site costs (patient cost), clinical research organization costs, and costs for central laboratory testing and data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through cost. These inputs are required to be estimated due to a lag in receiving the actual clinical information from third parties. . Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. Leases Effective January 1, 2019, the Company adopted ASU 2016-2, the details of which are further discussed in Note J. The Company determines if an arrangement is a lease at inception. Operating leases include right-of-use (ROU) assets and operating lease liabilities (current and non-current), which are recorded in the Company’s consolidated balance sheets. Single payment capital leases for equipment that are considered finance leases are included in property and equipment in the Company’s consolidated balance sheets. As the single payment obligations have all been made, there is no related liability recorded. ROU 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. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses the implicit rate when readily determinable. As a number of the Company’s leases do not provide an implicit rate, the Company uses an incremental borrowing rate applicable to the Company based on the information available at the commencement date in determining the present value of lease payments. As the Company has no existing or proposed collateralized borrowing arrangements, to determine a reasonable incremental borrowing rate, the Company considers collateral assumptions, the lease term, the Company’s current credit risk profile, and rates for existing borrowing arrangements for comparable peer companies. The operating lease ROU assets were netted against any lease incentive and straight-line lease liability balances at January 1, 2019 upon adoption of ASC 842. The Company accounts for the lease and fixed non-lease components as a single lease component. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term. Other Accrued Liabilities Other accrued liabilities consisted of the following at December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, December 31, ​ 2020 ​ 2019 Accrued contract payments ​ $ 15,576 ​ $ 5,188 Accrued clinical trial costs ​ 11,401 ​ 6,418 Accrued professional services ​ 1,200 ​ 1,274 Accrued employee benefits ​ 39 ​ 314 Accrued public reporting charges ​ 319 ​ 180 Other current accrued liabilities ​ 785 ​ 558 Total ​ $ 29,320 ​ $ 13,932 ​ Research and Development Expenses The Company’s research and development expenses are charged to expense as incurred and relate to (i) research to evaluate new targets and to develop and evaluate new antibodies, linkers, and cytotoxic agents, (ii) preclinical testing of its own and, in certain instances, its collaborators’ product candidates, and the cost of its own clinical trials, (iii) development related to clinical and commercial manufacturing processes, and (iv) external manufacturing operations and, prior to 2019, internal manufacturing operations, which also included raw materials. Payments made by the Company in advance for research and development services not yet provided and/or materials not yet delivered and accepted are recorded as prepaid expenses and are included in the accompanying consolidated balance sheets as prepaid and other current assets. Income Taxes The Company uses the liability method to account for income taxes. Deferred tax assets and liabilities are determined based on differences between the financial reporting and income tax basis of assets and liabilities, as well as net operating loss carry forwards and tax credits and are measured using the enacted tax rates and laws that will be in effect when the differences reverse. A valuation allowance against net deferred tax assets is recorded if, based on the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. Property and Equipment Property and equipment are stated at cost. The Company provides for depreciation based upon expected useful lives using the straight-line method over the following estimated useful lives: ​ ​ ​ ​ ​ Machinery and equipment 5 years ​ Computer hardware and software 3 years ​ Furniture and fixtures 5 years ​ Leasehold improvements Shorter of remaining lease term or 7 years ​ ​ Equipment under capit

Agreements

Agreements	12 Months Ended
	Dec. 31, 2020
Agreements
Agreements	C. Agreements Significant Collaborative Agreements Roche In 2000, the Company granted Genentech, now a unit of Roche, an exclusive development and commercialization license to use the Company’s maytansinoid ADC technology with antibodies, such as trastuzumab, or other proteins that target HER2. Under the terms of this agreement, Roche has exclusive worldwide rights to develop and commercialize maytansinoid ADC compounds targeting HER2. In 2013, the HER2-targeting ADC, Kadcyla, was approved for marketing in the U.S., Japan, and the European Union, or EU. Roche has also received marketing approval in various other countries around the world. Roche is responsible for the manufacturing, product development, and marketing of any products resulting from the agreement. The Company received a $2 million non-refundable upfront payment from Roche upon execution of the agreement. The Company is also entitled to receive up to a total of $44 million in development and regulatory milestone payments, plus royalties on the commercial sales of Kadcyla or any other resulting products. Through December 31, 2020, the Company has received and recognized $39.0 million in milestone payments related to Kadcyla. On May 3, 2019, Roche notified the Company that the FDA approved Kadcyla for adjuvant (after surgery) treatment of people with HER2-positive early breast cancer who have residual invasive disease after neoadjuvant (before surgery) taxane and Herceptin® (trastuzumab)-based treatment, resulting in a $5 million regulatory milestone payment to the Company for a first extended indication, which is included in license and milestone fees for the year ended December 31, 2019. The next and final potential milestone the Company will be entitled to receive will be a $5 million regulatory milestone for marketing approval of Kadcyla for a second extended indication as defined in the agreement. The Company receives royalty reports and payments related to sales of Kadcyla from Roche one Roche, through its Genentech unit, also has licenses for the exclusive right to use the Company’s maytansinoid ADC technology with antibodies to four undisclosed targets, which were granted under the terms of a separate, now expired, 2000 right-to-test agreement with Genentech. For each of these licenses, the Company received a $1 million license fee and is entitled to receive up to a total of $38 million in milestone payments and also royalties on the sales of any resulting products. The total milestones are categorized as follows: development and regulatory milestones—$28 million; and sales milestones—$10 million. The Company has not received any milestone payments from these agreements through December 31, 2020. Roche is responsible for the development, manufacturing, and marketing of any products resulting from these licenses. The next potential milestone the Company will be entitled to receive under any of these agreements will be a development milestone for filing of an IND application which will result in a $1 million payment being due. Amgen/Oxford BioTherapeutics Under a now-expired right-to-test agreement established in 2000, the Company granted Amgen four exclusive development and commercialization licenses, for which the Company received an exercise fee of $1 million for each license taken. Three of the four licenses have since been terminated by Amgen, and Amgen has sublicensed its rights under the one remaining license to Oxford BioTherapeutics Ltd. (OBT). For the remaining development and commercialization license, the Company is entitled to receive up to a total of $34 million in milestone payments, plus royalties on the commercial sales of any resulting products. The total milestones are categorized as follows: development and regulatory milestones—$29 million; and sales milestones—$5 million. Amgen (or its sublicensee(s)) is responsible for the manufacturing, development, and marketing of any products resulting from this development and commercialization license. Through December 31, 2020, the Company has received and recognized an aggregate of $4 million in milestone payments for compounds covered under this agreement now or in the past. The next potential milestone the Company will be entitled to receive under the remaining license will be a development milestone for the first dosing of a patient in a U.S. Phase 2 clinical trial, which will result in a $3 million payment being due. Bayer In 2008, the Company granted Bayer an exclusive development and commercialization license to the Company’s maytansinoid ADC technology for use with antibodies or other proteins that target mesothelin. The Company received a $4 million upfront payment upon execution of the agreement. For each compound developed and marketed by Bayer under this collaboration the Company is entitled to receive a total of $170.5 million in milestone payments, plus royalties on the commercial sales of any resulting products. The total milestones are categorized as follows: development and regulatory milestones—$60.5 million; and sales milestones—$110 million. Through December 31, 2020, the Company has received and recognized an aggregate of $13 million in milestone payments under this agreement. The next potential milestone the Company will be entitled to receive will be either a development milestone for commencement of a pivotal clinical trial for a second indication for anetumab ravtansine which will result in a $2 million payment being due or a regulatory milestone for filing of regulatory approval for its first indication for anetumab ravtansine which will result in a $6 million payment being due. Bayer is responsible for the research, development, manufacturing, and marketing of any products resulting from the license. Novartis The Company granted Novartis exclusive development and commercialization licenses to the Company’s maytansinoid and IGN ADC technology for use with antibodies to six specified targets under a now-expired right-to-test agreement established in 2010. The Company received a $45 million upfront payment in connection with the execution of the right-to-test agreement in 2010, $8.5 million in extension and amendment fees, and an exercise fee of $1 million for each of the six licenses taken. In May 2018, Novartis terminated one of its six licenses. As a result, the Company recorded the remaining unrecognized $1.0 million balance of the upfront payment that had been allocated to future performance obligations under this license as revenue, which is included in license and milestone fees for the year ended December 31, 2018. For the remaining development and commercialization licenses, the Company is entitled to receive up to a total of $199.5 million in milestone payments, plus royalties on the commercial sales of any resulting products. The total milestones are categorized as follows: development and regulatory milestones—$99.5 million; and sales milestones—$100 million. In 2015 and 2016, Novartis initiated Phase 1 testing of three of its five product candidates, triggering a $5 million development milestone payment to the Company with each event. Novartis later discontinued clinical testing of these three products. In December 2019, a development milestone related to dosing a first patient in a Phase 1 clinical trial for a separate licensed product became probable of being attained. Accordingly, $4.7 million of the $5.0 million milestone that was allocated to the delivered license was recorded as revenue and is included in license and milestone fees for the year ended December 31, 2019, and $0.3 million that was allocated to future technological improvements was deferred and will be recognized as revenue ratably over the estimated term of the license. In September 2020, Novartis enrolled its first patient in the aforementioned Phase 1 clinical trial and remitted the $5.0 million milestone payment to the Company. The next potential payment the Company could receive would be either a $7.5 million development milestone for commencement of a Phase 2 clinical trial or a $5 million development milestone for commencement of a Phase 1 clinical trial. Novartis is responsible for the manufacturing, development, and marketing of any products resulting from this agreement. CytomX In 2016, the Company granted CytomX an exclusive development and commercialization license to the Company’s maytansinoid ADC technology for use with Probodies™ that target CD166 under a now expired reciprocal right-to-test agreement. The Company neither received nor made an upfront cash payment in connection with the execution of the right-to-test agreement or the license agreement. An amendment of the right-to-test agreement executed simultaneously with the license granted CytomX the right, for a specified period of time, to substitute the specified target with another as yet unspecified target. Accordingly, the revenue associated with this license was deferred until the expiration of that substitution right in January 2017, whereupon the Company recognized $12.7 million of the $13 million of arrangement consideration allocated to the development and commercialization license. With respect to the development and commercialization license granted to CytomX, the Company is entitled to receive up to a total of $160 million in milestone payments plus royalties on the commercial sales of any resulting product. The total milestones are categorized as follows: development and regulatory milestones—$60 million; and sales milestones—$100 million. In June 2017, CytomX enrolled its first patient in a Phase 1 clinical trial for its product candidate, CX-2009, triggering a $1 million development milestone payment. In December 2019, a development milestone related to dosing of a first patient in a Phase 2 clinical trial became probable of being attained, which resulted in $3.0 million of license and milestone fee revenue being recorded in 2019. In February 2020, CytomX notified the Company that it had enrolled its first patient in the aforementioned Phase 2 clinical trial. The next payment the Company could receive would be a $6.0 million development milestone payment with commencement of a Phase 3 clinical trial. CytomX is responsible for the manufacturing, development, and marketing of any products resulting from the development and commercialization license taken by CytomX under this collaboration. Costs directly attributable to the CytomX collaborative agreement are comprised of compensation and benefits related to employees who provided research and development services on behalf of CytomX as well as costs of clinical materials sold. Indirect costs are not identified to individual collaborators. For the year ended December 31, 2018, the costs related to the research and development services and clinical materials sold amounted to $0.2 million and $3.5 million. There were no similar costs recorded subsequently. In 2017, the Company took exclusive development and commercialization licenses to CytomX’s proprietary antibody-masking (Probody) technology for use with Probodies that target two specified targets under the same reciprocal right-to-test agreement. The Company terminated one of these licenses for convenience prior to the end of 2017 and terminated the second license in December 2019 in connection with the grant of the EpCAM license to CytomX discussed further below. No upfront cash payments were made by the Company with the execution of these license agreements. The arrangement was accounted for based on the fair value of the items exchanged. The items to be delivered to CytomX under the arrangement are accounted for under the Company’s revenue recognition policy. The items that were received from CytomX were recorded as research and development expenses as incurred. In December 2019, the Company granted CytomX an exclusive development and commercialization license to maytansinoid and IGN ADC technology for use with Probodies™ that target EpCAM. Pursuant to the license agreement, in January 2020, the Company received a $7.5 million upfront payment, of which $7.3 million was recorded as license and milestone fee revenue upon delivery of the license to CytomX in December 2019 and $0.2 million was deferred until delivery of certain materials as these performance obligations were determined to be distinct. The Company is also entitled to receive up to a total of $355 million in milestone payments plus royalties on the commercial sales of any resulting product. The total milestones are categorized as follows: development and regulatory milestones—$205 million; and sales milestones—$150 million. CytomX is responsible for the manufacturing, product development, and marketing of any products resulting from this license. Fusion In December 2016, the Company entered into an exclusive license agreement to a specified target with Fusion Pharmaceuticals Inc. The Company is entitled to receive up to a total of $50 million in milestone payments plus royalties on the commercial sales of any resulting products. The total milestones are categorized as follows: development and regulatory milestones—$15 million; and sales milestones—$35 million. During the year ended December 31, 2018, a development milestone related to dosing of a first patient in a Phase I clinical trial became probable of being attained, which resulted in a $0.5 million contract asset and the related license and milestone fee revenue being recorded in that year. It was subsequently paid in 2019. The next potential milestone payment the Company will be entitled to receive will be a $1.5 million development milestone payment with the initiation of a Phase II clinical trial. Fusion is responsible for the manufacturing, development, and marketing of any products resulting from the license. Debiopharm In May 2017, Debiopharm International SA (Debiopharm) acquired the Company’s IMGN529 program, a clinical-stage anti-CD37 ADC for the treatment of patients with B-cell malignancies, such as non-Hodgkin lymphomas (NHL). Under the terms of the Exclusive License and Asset Purchase agreement, the Company received a $25 million upfront payment for specified assets related to IMGN529, a paid-up license to the Company’s ADC technology and a $5 million milestone payment upon substantial completion of the transfer of ImmunoGen technologies related to the program (technology transfer). This technology transfer was completed in the fourth quarter of 2017, and $4.5 million was received for this milestone in December 2017, and the $0.5 million balance in January 2018 upon delivery of the final materials related to the transfer. Accordingly, the Company recorded $0.5 million and $29.5 million of license and milestone fee revenue in 2018 and 2017, respectively. In addition, ImmunoGen is eligible for a second success-based milestone payment of $25 million upon IMGN529 entering a Phase 3 clinical trial. The milestone payment will be significantly reduced if a Phase 3 trial using the Company’s technology but not the IMGN529 antibody commences prior to IMGN529 entering a Phase 3 trial. The Company does not believe this scenario is likely to occur. Viridian In October 2020, the Company entered into a license agreement with Viridian Therapeutics, Inc. pursuant to which the Company granted Viridian the exclusive right to develop and commercialize an insulin-like growth factor-1 receptor (IGF-1R) antibody for all non-oncology indications that do not use radiopharmaceuticals in exchange for an upfront payment, with the potential to receive up to a total of $143.0 million in milestone payments plus royalties on the commercial sales of any resulting product. The total milestones are categorized as follows: development and regulatory milestones—$48.0 million; and sales milestones—$95.0 million. Viridian is responsible for the manufacturing, development, and marketing of any products resulting from the license agreement. Huadong In October 2020, the Company entered into a collaboration and license agreement with Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), a subsidiary of Huadong Medicine Co., Ltd. The collaboration and license agreement grants Huadong an exclusive, royalty-bearing, and sublicensable right to develop and commercialize mirvetuximab (the Licensed Product) in the People’s Republic of China, Hong Kong, Macau, and Taiwan (collectively, Greater China). The Company retains exclusive rights to the Licensed Product outside of Greater China. Under the terms of the collaboration and license agreement, the Company received a non-refundable upfront payment of $40.0 million with the potential for approximately $265.0 million in milestone payments. The total milestones are categorized as follows: development and regulatory milestones—$80.0 million; and sales milestones—$185.0 million. In addition, the Company is entitled to receive tiered percentage royalties ranging from low double digits to high teens as a percentage of commercial sales of the Licensed Product, if approved, by Huadong in Greater China, subject to adjustment in specified circumstances. The Company evaluated the agreement and determined it was within the scope of ASC 606. The Company determined the promised goods and services included the license to intellectual property and know-how and the clinical supply of the Licensed Product to Huadong for a specified period. The Company concluded that the license to intellectual property and know-how is not distinct from the clinical supply of the Licensed Product because the clinical supply is essential to the use of the license and an alternative source of clinical supply is not readily available in the marketplace. Accordingly, these two promised goods and services are considered a single combined performance obligation. The Company determined there were no options in the agreement that represented material rights. The transaction price was determined to consist of the upfront payment of $40.0 million and estimated payments to be received for clinical supply of the Licensed Product. Future development and regulatory milestones have been fully constrained. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Huadong. The Company re-evaluates the transaction price, including its estimated variable consideration included in the transaction price and all constrained amounts, at each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company determined that revenue related to the agreement would be recognized as the clinical supply of the Licensed Product is delivered to Huadong, estimated to be completed over approximately two years. The Company has estimated the total clinical supply to be delivered during this time and will reassess the percentage of clinical supply that has been delivered on an ongoing basis. If a change in estimate is determined to be necessary, the Company will adjust revenue using a cumulative catch-up method. No revenue related to this agreement has been recognized in the year ended December 31, 2020. Terminated Agreements Jazz Pharmaceuticals In August 2017, the Company entered into a Collaboration and Option Agreement (the “Option Agreement”) with Jazz Pharmaceuticals Ireland Limited (Jazz), a subsidiary of Jazz Pharmaceuticals plc, granting Jazz exclusive, worldwide rights to opt into development and commercialization of two early-stage, hematology-related ADC programs, as well as an additional program to be designated during the term of the agreement. The programs covered under the agreement included IMGN779, a CD33-targeted ADC for the treatment of acute myeloid leukemia (AML) then in Phase 1 testing, IMGN632, a CD123-targeted ADC for hematological malignancies also then in Phase 1 testing, and an early-stage program to be determined at a later date. As part of the Option agreement, Jazz made an upfront payment of $75 million to the Company. Additionally, Jazz had also agreed to pay the Company up to $100 million in development funding over seven years to support the three ADC programs. In October 2019, Jazz exercised certain opt-out rights under the Option Agreement following the termination of the Company’s IMGN779 development program. In addition, in November 2019, the Company executed a First Amendment (the “First Amendment”) to the Option Agreement. The First Amendment included an exercise of Jazz’s opt-out rights related to the termination of the Company’s early research programs covered by the Option Agreement in connection with the Company’s previously announced restructuring. Under the terms of the Option Agreement, the exercise of both of these opt-out rights resulted in a pro-rata reduction in Jazz’s obligation to provide development funding, with support being limited to the Company’s IMGN632 development program. In December 2020, the Company received notice that, based on the outcome of an internal portfolio review, Jazz exercised its opt out rights with respect to IMGN632, thereby relinquishing the development and commercialization option. As a result of Jazz’s opting out, the Company retains all rights to IMGN632 and is continuing global development of IMGN632 without further involvement by Jazz, except that Jazz will continue to provide a predetermined amount of research funding for the IMGN632 program over the next twelve months. Due to the timing of the Jazz opt out, the Company will not owe royalty payments to Jazz on commercial sales of IMGN632 if it is approved. Due to the involvement the Company and Jazz both had in the development and commercialization of the products, as well as both parties being part of the cost share agreement and exposed to significant risks and rewards dependent on the commercial success of the products, the arrangement was determined to be a collaborative arrangement within the scope of ASC 808. Accordingly, the Company carved out the research and development activities and the related cost sharing arrangement with Jazz. Payments for such activities are recorded as research and development expense and reimbursements received from Jazz are recognized as an offset to research and development expense in the accompanying statement of operations during the development period. Included in research and development expense for the years ended December 31, 2020, 2019 and 2018, are $6.7 million, $12.5 million, and $10.0 million of credits related to reimbursements from Jazz, respectively. The non-refundable, upfront arrangement consideration of $75 million was allocated to the three license options. The amount allocated to the rights to future technological improvements under the relative selling price method was deemed immaterial and, therefore, not segregated from the license options. In conjunction with the opt-out of IMGN779, the Company recognized $14.5 million of the deferred revenue in the year ended December 31, 2019. In connection with the execution of the First Amendment, the amount of the transaction price originally allocated to the early research product Options was reallocated to the to the IMGN632 Option, which represented the only remaining material right. The remaining $60.5 million of previously deferred license revenue was recognized upon the opt-out of the right to execute the last license by Jazz in December 2020. Takeda In March 2015, the Company entered into a three-year right-to-test agreement with Takeda Pharmaceutical Company Limited (Takeda) through its wholly-owned subsidiary, Millennium Pharmaceuticals, Inc., pursuant to which the Company received a $20 million upfront payment. A first license was granted to Takeda in 2015, whereupon the Company recognized $8.6 million of the arrangement consideration allocated to the development and commercialization licenses. In 2018, the right-to-test agreement expired without Takeda exercising its option to a second license. Accordingly, the remaining $10.9 million of revenue that had been deferred for such performance obligations was recognized as revenue and is included in license and milestone fees for the year ended December 31, 2018. In May 2018, Takeda enrolled its first patient in a Phase 1 clinical trial, triggering a $5.0 million milestone payment to the Company. Due to the likelihood of this milestone being attained, this milestone was recognized as a contract asset as part of the cumulative adjustment to transition to ASC 606. It had been previously allocated to the delivered license and the right to technological improvements. In 2020, Takeda terminated its exclusive development and commercialization license. As a result, the Company recorded the remaining $0.9 million balance of the upfront payment that had been allocated to future performance obligations under the license as revenue, which is included in license and milestone fees for the year ended December 31, 2020. Biotest In 2006, the Company granted Biotest an exclusive development and commercialization license to the Company’s maytansinoid ADC technology for use with antibodies that target CD138, pursuant to which the Company received a $1 million upfront payment. In 2020, Biotest terminated the license. Lilly Under a now-expired right-to-test agreement established in 2011, the Company granted Eli Lilly and Company (Lilly) three exclusive development and commercialization licenses, for which the Company received a $20 million upfront payment in connection with the execution of the right-to-test agreement and exercise fees totaling $4 million for the three licenses taken. In October 2018, Lilly terminated its three development and commercialization licenses. As a result, the Company recorded the remaining unrecognized $0.7 million balance of the upfront payment that had been allocated to future performance obligations under this license as revenue, which is included in license and milestone fees for the year ended December 31, 2018. ​

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2020
Property and Equipment
Property and Equipment	D. Property and Equipment Property and equipment consisted of the following at December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2020 ​ 2019 ​ Leasehold improvements ​ $ 21,890 $ 20,776 ​ Machinery and equipment ​ 2,861 ​ 9,384 ​ Computer hardware and software ​ 5,636 ​ 5,692 ​ Furniture and fixtures ​ 3,039 ​ 3,607 ​ Assets under construction ​ 97 ​ — ​ ​ ​ $ 33,523 ​ $ 39,459 ​ Less accumulated depreciation ​ (27,763) ​ (32,466) ​ Property and equipment, net ​ $ 5,760 ​ $ 6,993 ​ ​ Included in the table above are amounts capitalized for equipment under capital leases at December 31, 2020 and 2019 totaling $2.1 million, net of accumulated amortization of $1.3 million and $1.0 million, respectively. Depreciation expense was $2.1 million, $4.0 million , and $7.4 million for the years ended December 31, 2020, 2019 and 2018, respectively. As a result of the restructuring at the end of the second quarter of 2019, the Company recorded an impairment charge of $2.5 million to write down excess equipment to fair value. During the fourth quarter of 2019, the Company executed an agreement to liquidate the equipment and transferred title to assets with a cost basis of $14.2 million and accumulated depreciation of $12.9 million, for which the Company received a $2 million payment. During the year ended December 31, 2020, the Company liquidated the remaining equipment with a cost basis of $6.7 million for an additional $1.2 million payment to the Company.

Convertible 4.5% Senior Notes

Convertible 4.5% Senior Notes	12 Months Ended
	Dec. 31, 2020
Convertible 4.5% Senior Notes
Convertible 4.5% Senior Notes	E. Convertible 4.5% Senior Notes In 2016, the Company issued convertible notes with an aggregate principal amount of $100 million, of which $2.1 million remains outstanding as of December 31, 2020. The convertible notes are governed by the terms of an indenture between the Company, as issuer, and Wilmington Trust, National Association, as the trustee. The convertible notes are senior unsecured obligations and bear interest at a rate of 4.5% per year, payable semi-annually in arrears on January 1 and July 1 of each year, commencing on January 1, 2017. The Company recorded $0.1 million of interest expense in each of the years ended December 31, 2020, 2019 and 2018, respectively. The convertible notes will mature on July 1, 2021, unless earlier repurchased or converted. Holders may convert their notes at their option at any time prior to the close of business on the business day immediately preceding the stated maturity date. Upon conversion, the Company will deliver for each $1,000 principal amount of converted notes a number of shares equal to the conversion rate, which will initially be 238.7775 shares of common stock, equivalent to an initial conversion price of approximately $4.19. The conversion rate will be subject to adjustment in some circumstances but will not be adjusted for any accrued and unpaid interest. The Company analyzed the terms of the convertible notes and determined that under current accounting guidance the notes would be entirely accounted for as debt and none of the terms of the notes require separate accounting.

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	12 Months Ended
	Dec. 31, 2020
Liability Related to Sale of Future Royalties
Liability Related to Sale of Future Royalties	F. Liability Related to Sale of Future Royalties In 2015, IRH purchased the right to receive 100% of the royalty payments on commercial sales of Kadcyla arising under the Company’s development and commercialization license with Genentech, until IRH had received aggregate royalties equal to $235 million or $260 million, depending on when the aggregate royalties received by IRH reached a specified milestone. Once the applicable threshold was met, if ever, the Company would thereafter have received 85% and IRH would have received 15% of the Kadcyla royalties for the remaining royalty term. At consummation of the transaction the Company received cash proceeds of $200 million. As part of this sale, the Company incurred $5.9 million of transaction costs, which are presented net of the liability in the accompanying consolidated balance sheet and are being amortized to interest expense over the estimated life of the royalty purchase agreement. Although the Company sold its rights to receive royalties from the sales of Kadcyla, as a result of its ongoing involvement in the cash flows related to these royalties, the Company continues to account for these royalties as revenue and recorded the $200 million in proceeds from this transaction as a liability related to sale of future royalties (Royalty Obligation) that will be amortized using the interest method over the estimated life of the royalty purchase agreement. In January 2019, the Company sold its residual rights to receive royalty payments on commercial sales of Kadcyla to OMERS, the defined benefit pension plan for municipal employees in the Province of Ontario, Canada, for a net payment of $65.2 million (amount is net of $1.5 million in broker fees). Simultaneously, OMERS purchased IRH’s right to the royalties the Company previously sold as described above, therefore obtaining the rights to 100% of the royalties received from that date on. Because the Company will not be involved with the cash flows related to the residual royalties, the $65.2 million of net proceeds received from the sale of its residual rights to receive royalty payments was recorded as deferred revenue and will be amortized as the royalty revenue related to the residual rights is earned using the units of revenue approach. Through December 31, 2020, no revenue related to the residuals rights was recognized. Additionally, the purchase of IRH’s interest by OMERS did not result in an extinguishment or modification of the original instrument and, accordingly, the Company will continue to account for the remaining obligation as a liability as outlined above. The following table shows the activity within the liability account during the year ended December 31, 2020 and the period from inception (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Period from ​ ​ Year Ended ​ inception to ​ December 31, 2020 ​ December 31, 2020 Liability related to sale of future royalties, net — beginning balance ​ $ 123,541 ​ $ — Proceeds from sale of future royalties, net ​ — ​ 194,135 Kadcyla royalty payments received and paid ​ (61,195) ​ (206,367) Non-cash interest expense recognized ​ ​ 23,093 ​ ​ 97,671 Liability related to sale of future royalties, net — ending balance ​ $ 85,439 ​ $ 85,439 ​ As royalties are remitted to IRH and subsequently OMERS, the balance of the Royalty Obligation will be effectively repaid over the life of the agreement. In order to determine the amortization of the Royalty Obligation, the Company is required to estimate the total amount of future royalty payments to be received and remitted as noted above over the life of the agreement. The sum of these amounts less the $200 million proceeds the Company received will be recorded as interest expense over the life of the Royalty Obligation. Since inception, the Company’s estimate of this total interest expense results in an imputed annual interest rate of 10.5% and a current imputed interest rate of 22.2% as of December 31, 2020. The Company periodically assesses the estimated royalty payments to IRH/OMERS and to the extent such payments are greater or less than its initial estimates, or the timing of such payments is materially different than its original estimates, the Company will prospectively adjust the amortization of the Royalty Obligation. There are a number of factors that could materially affect the amount and timing of royalty payments from Genentech, most of which are not within the Company’s control. Such factors include, but are not limited to, changing standards of care, the introduction of competing products, manufacturing or other delays, biosimilar competition, patent protection, adverse events that result in governmental health authority imposed restrictions on the use of the drug products, significant changes in foreign exchange rates as the royalties remitted to IRH/OMERS are made in U.S. dollars (USD) while significant portions of the underlying sales of Kadcyla are made in currencies other than USD, and other events or circumstances that could result in reduced royalty payments from Kadcyla, all of which would result in a reduction of non-cash royalty revenues and the non-cash interest expense over the life of the Royalty Obligation. Conversely, if sales of Kadcyla are more than expected, the non-cash royalty revenues and the non-cash interest expense recorded by the Company would be greater over the term of the Royalty Obligation. In addition, the royalty purchase agreement grants IRH/OMERS the right to receive certain reports and other information relating to the royalties and contains other representations and warranties, covenants, and indemnification obligations that are customary for a transaction of this nature.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Taxes
Income Taxes	G. Income Taxes The difference between the Company’s expected tax benefit, as computed by applying the applicable U.S. federal corporate tax rate to loss before the benefit for income taxes, and actual tax is reconciled in the following chart (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, ​ ​ 2020 2019 ​ 2018 Loss before income tax expense $ (44,372) $ (104,133) $ (168,843) Expected tax benefit at 21% $ (9,318) ​ $ (21,868) ​ $ (35,457) Permanent differences ​ 157 ​ 320 ​ (103) Incentive stock options ​ ​ 201 ​ ​ 569 ​ ​ 1,144 State tax benefit net of federal benefit ​ (2,250) ​ (6,726) ​ (10,622) Change in valuation allowance, net ​ 15,175 ​ 27,812 ​ 53,706 Federal research credit ​ (228) ​ (1,652) ​ (2,466) Federal orphan drug credit ​ ​ (6,218) ​ ​ (4,426) ​ ​ (6,934) Expired loss and credit carryforwards ​ ​ 419 ​ ​ 500 ​ ​ — Lease incentive ​ ​ — ​ ​ — ​ ​ 109 Stock option expirations ​ 2,062 ​ 5,471 ​ 623 Benefit for income taxes ​ $ — ​ $ — ​ $ — ​ At December 31, 2020, the Company has net operating loss, or NOL, carryforwards of $471.6 million available to reduce federal taxable income, if any, that begin to expire in 2028 through 2037 and $373.7 million of the federal NOL carryforwards can be carried forward indefinitely. The Company has $677.1 million of NOL carryforwards available to reduce state taxable income, if any, that expire in 2033 through 2040. The Company also has federal and state credit carryforwards of $70.4 million and $13.0 million, respectively, available to offset federal and state income taxes, which expire beginning in 2022. Due to the degree of uncertainty related to the ultimate use of the loss carryforwards and tax credits, the Company has established a valuation allowance to fully reserve these tax benefits. Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2020 and 2019 are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 ​ 2019 Deferred tax assets: ​ ​ ​ ​ Net operating loss carryforwards $ 220,286 ​ $ 191,744 Research and development tax credit carryforwards ​ 80,694 ​ 75,084 Property and other intangible assets ​ 871 ​ 809 Deferred revenue ​ 30,082 ​ 36,008 Stock-based compensation ​ 9,940 ​ 9,630 Operating lease liability ​ 6,033 ​ 6,767 Other liabilities ​ 1,514 ​ 2,255 Royalty sale ​ 17,455 ​ ​ 30,030 Total deferred tax assets $ 366,875 ​ $ 352,327 ​ ​ ​ ​ ​ ​ Deferred tax liabilities: ​ ​ ​ ​ ​ Stock-based compensation ​ (58) ​ ​ (110) Operating lease right of use asset ​ (3,844) ​ ​ (4,258) Royalty sale transaction costs ​ (247) ​ ​ (408) Total deferred tax liabilities $ (4,149) ​ $ (4,776) Valuation allowance ​ (362,726) ​ (347,551) Net deferred tax assets/(liabilities) $ — ​ $ — The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. The Company has determined that it is not more-likely-than-not that the tax benefits related to the federal and state deferred tax assets will be realized for financial reporting purposes. Accordingly, the deferred tax assets have been fully reserved at December 31, 2020 and 2019. The valuation allowance increased by $15.2 million during the year ended December 31, 2020 due primarily to additional net loss incurred during the year. Utilization of the NOL and credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that have occurred previously or that could occur in the future as provided by Sections 382 and 383 of the Internal Revenue Code of 1986, as well as similar state and foreign provisions. These ownership changes may limit the amount of NOL and credit carry forwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50 percentage points over a three-year period. Since the Company’s formation, it has raised capital through the issuance of capital stock on several occasions (both pre and post initial public offering) which, combined with the purchasing shareholders’ subsequent disposition of those shares, may have resulted in a change of control, as defined by Section 382, or could result in a change of control in the future upon subsequent disposition. During fiscal year 2015, the Company completed a study to assess whether a change of control has occurred or whether there have been multiple changes of control since its formation and determined no ownership change occurred under Section 382. This study was updated through December 31, 2020 resulting in the same conclusion. Additionally, the Company has not completed a detailed Research and Development Credit Study (including the Orphan Drug Credit); accordingly, a portion of the tax credit carryforward may not be available to offset future income. The Company accounts for uncertain tax positions under the recognition and measurement criteria of ASC 740-10. For those tax positions for which 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% likelihood of being realized upon settlement with a taxing authority that has full knowledge of all relevant information. If the Company does not believe that it is not more likely than not that a tax benefit will be sustained, no tax benefit is recognized. As of December 31, 2020 and 2019, no uncertain tax positions have been recorded. Interest and penalties related to the settlement of uncertain tax positions, if any, will be reflected in income tax expense. The Company did not recognize any interest and penalties associated with unrecognized tax benefits in the accompanying consolidated financial statements. The Company does not expect any material changes to the unrecognized benefits within 12 months of the reporting date. Due to existence of the valuation allowance, future changes in the Company’s unrecognized tax benefits will not impact its effective tax rate. The statute of limitations for assessment by the Internal Revenue Service, or IRS, and state tax authorities is open for tax years ending after December 31, 2017, although carryforward attributes that were generated prior to 2017 may still be adjusted upon examination by the IRS or state tax authorities if they either have been or will be used in a future period.

Capital Stock

Capital Stock	12 Months Ended
	Dec. 31, 2020
Capital Stock
Capital Stock	H. Capital Stock Common Stock Reserved At December 31, 2020, the Company has reserved 25.0 million shares of authorized common stock for the future issuance of shares under the 2018, ESPP and Inducement Plans. See “Stock-Based Compensation” in Note B for a description of the 2018, ESPP, and Inducement Plans. Stock Options As of December 31, 2020, the 2018 Plan and the Inducement Plan were the only employee share-based compensation plans of the Company under which grants can be made. During the year ended December 31, 2020, holders of options issued under the option plans exercised their rights to acquire an aggregate of 336,000 shares of common stock at prices ranging from $2.31 to $5.25 per share. The total proceeds to the Company from these option exercises were $1.0 million. The Company granted options with an exercise price equal to the fair market value of the common stock on the date of such grant. The following options and their respective weighted-average exercise prices per share were exercisable at December 31, 2020, 2019, and 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ‑ ​ ​ Exercisable ​ Average ​ ​ (in thousands) ​ Exercise Price December 31, 2020 6,983 ​ $ 8.15 ​ December 31, 2019 5,801 ​ $ 10.16 ​ December 31, 2018 8,405 ​ $ 11.47 ​ ​ 2004 Non-Employee Director Compensation and Deferred Share Unit Plan Under the 2004 Non-Employee Director Compensation and Deferred Share Unit Plan, or the 2004 Director Plan, as amended, between 2004 and 2009 non-employee directors were paid their annual retainers in the form of deferred stock units, based on the fair market value of the Company’s common stock on the last date of the Company’s fiscal year prior to the year for which services were rendered, and in cash, with the option, at their discretion, to have all or a portion of the cash portion paid in additional deferred stock units. All deferred stock units awarded under the 2004 Director Plan have vested and are redeemed on the date a director ceases to be a member of the Board, at which time such director’s deferred stock units will be settled in shares of common stock of the Company issued under the 2006 Plan at a rate of one share for each vested unit. Compensation Policy for Non-Employee Directors In September 2009, the Board adopted a new Compensation Policy for Non-Employee Directors, which superseded the 2004 Plan and made certain changes to the compensation of its non-employee directors. The Compensation Policy for Non-Employee Directors, as amended as of June 2020, consists of three elements: cash compensation; deferred stock units; and stock options. Cash Compensation Each non-employee director receives annual meeting fees which are paid in quarterly installments in, at each director’s election, either cash or deferred stock units. Deferred Stock Units Pursuant to the Compensation Policy for Non-Employee Directors, as amended, non-employee directors receive deferred stock units upon initial election to the Board and annually thereafter. Vested deferred stock units are redeemed on the date a director ceases to be a member of the Board, at which time such director’s deferred stock units will generally be settled in shares of the Company’s common stock issued under our 2018 Plan (or its predecessor plans, depending on the grant date of the deferred stock units) at a rate of one share for each vested deferred stock unit then held. Any deferred stock units that remain unvested at that time will be forfeited. Pursuant to the Compensation Policy for Non-Employee Directors, in 2018, the Company issued retiring directors 172,509 shares of common stock of the Company to settle outstanding deferred share units. Pursuant to the Compensation Policy for Non-Employee Directors, as amended, the Company recorded: ● $0.3 million in compensation expense during the year ended December 31, 2020 related to the grant of 127,000 deferred share units and 15,000 deferred share units previously granted; ● $0.3 million in compensation expense during the year ended December 31, 2019 related to the grant of 63,000 deferred share units and 18,000 deferred share units previously granted; and ● $0.4 million in compensation expense during the year ended December 31, 2018 related to the grant of 46,000 deferred share units and 10,500 deferred share units previously granted. Stock Options Pursuant to the Compensation Policy for Non-Employee Directors, as amended, non-employee directors also receive stock option awards upon initial election to the Board and annually thereafter. The directors received a total of 300,000, 108,000, and 128,000 options in the years ended December 31, 2020, 2019, and 2018, and the related stock compensation expense is included in the amounts discussed in the “Stock-Based Compensation” section of footnote B above.

Restructuring Charges

Restructuring Charges	12 Months Ended
	Dec. 31, 2020
Restructuring Charge
Restructuring Charges	I. Restructuring Charge 2019 Corporate Restructuring On June 26, 2019, the Board of Directors approved a plan to restructure the business to focus resources on continued development of mirvetuximab and a select portfolio of three earlier-stage product candidates, resulting in a significant reduction of our workforce, with a majority of these employees separating from the business by mid-July 2019 and most of the remaining affected employees transitioning over varying periods of time of up to 12 months. Communication of the plan to the affected employees was substantially completed on June 27, 2019. As a result of the workforce reduction, during the three months ended June 30, 2019, the Company recorded a Compensation-Nonretirement Postemployment Benefits A summary of activity against the corporate restructuring charge related to the employee terminations in 2019 is as follows: ​ ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2019 ​ $ 4,087 Additional charges/adjustments during the period ​ ​ (116) Payments during the period ​ ​ (3,187) Balance at December 31, 2020 ​ $ 784 ​ In addition to the termination benefits and other related charges, the Company has sub-leased laboratory and office space at 830 Winter Street in Waltham, Massachusetts no longer used in the business. The decision to vacate part of its corporate office resulted in a change in asset groupings and also represented an impairment indicator. The Company determined and continues to believe that the right-of-use asset and leasehold improvements are recoverable based on expected sublease income, and therefore, no impairment has been recorded. In addition, the Company also decided to liquidate excess laboratory equipment and expected the proceeds to be less than the carrying value. As a result, in 2019, the Company recorded an impairment charge of $2.5 million to write down the equipment to fair value based on current market re-sale estimates obtained. 2018 Manufacturing Restructuring In February 2018, following an in-depth review of manufacturing and quality operations, the Board of Directors authorized management to implement a new operating model that will rely on external manufacturing and quality testing for drug substance and drug product for the Company’s development programs. The implementation of this new operating model led to the ramp-down of manufacturing and quality activities at the Norwood, Massachusetts facility by the end of 2018, and a full decommissioning of the facility in February 2019. Implementation of the new operating model resulted in the separation of 22 employees. Communication of the plan to the affected employees was substantially completed on February 8, 2018. In connection with the implementation of the new operating model, the Company recorded a one-time charge of $1.2 million for severance related to a pre-existing plan in the first quarter of 2018 in accordance with ASC 712, Compensation-Nonretirement Postemployment Benefits, as such amounts were probable and reasonably estimable. Additional expense was recorded for incremental retention benefits over the remaining service period of the related employees, as well as marginal adjustments to severance resulting from voluntary terminations, which totaled $2.3 million for the remainder of 2018. Cash payments related to retention benefits were paid in the fourth quarter of 2018 and those related to severance were paid out by the end of the third quarter of 2019. Additionally, certain options held by the employees to be separated were modified to extend the exercise period, resulting in a stock compensation charge of $0.2 million in the first quarter of 2018. Charge Related to Unoccupied Office Space The Company has sought to sub-lease 10,281 square feet of unoccupied office space at 930 Winter Street in Waltham, Massachusetts that was leased in 2016. During 2019, the Company recorded a $0.6 million impairment charge related to this lease, which represented the remaining balance of the right to use asset as the likelihood of finding a sub-lessor had diminished significantly as the lease approaches termination.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases
Leases	J. Leases Leases The Company currently has two real estate leases. The first is an agreement with CRP/King 830 Winter L.L.C. for the rental of approximately 120,000 square feet of laboratory and office space at 830 Winter Street, Waltham, Massachusetts through March 2026. The Company uses this space for its corporate headquarters and other operations. The Company may extend the lease for two additional terms of five years and is required to pay certain operating expenses for the leased premises subject to escalation charges for certain expense increases over a base amount. During 2020, the Company executed four subleases for approximately 65,000 square feet through the remaining initial term of the lease. The balance of the space will be used by the Company. The second real estate lease is an agreement with PDM 930 Unit, LLC for the rental of 10,281 square feet of additional office space at 930 Winter Street, Waltham, Massachusetts through August 31, 2021. The Company is required to pay certain operating expenses for the leased premises based on its pro-rata share of such expenses for the entire rentable space of the building. The Company ended its lease and vacated its manufacturing and office space at 333 Providence Highway, Norwood, Massachusetts in February 2019 pursuant to the restructuring plan described previously. In addition to the two real estate leases noted above, the Company currently has a lease agreement through November 2023 for the rental of copier equipment. During the first quarter of 2019, the Company adopted ASC 842 by recognizing and measuring leases existing at, or entered into after, January 1, 2019. In accordance with the transition method provided by ASC 2018-11, the Company adopted and initially applied the new leasing rules on January 1, 2019, rather than at the earliest comparative period presented in the financial statements. Therefore, prior periods presented are in accordance with the previous lease guidance (ASC 840). As permitted by the new lease standard, the Company elected to apply the following practical expedients to the entire lease portfolio: (i) not to reassess whether any expired or existing contracts are or contain leases or the classification of any expired or existing leases; (ii) not to apply the recognition requirements to short-term leases; and (iii) not to separate fixed nonlease components from associated lease components for the underlying assets. Upon adoption, a ROU asset of $17.6 million and a lease liability of $27.3 million were recorded and are identified separately in the Company’s consolidated balance sheets for the existing operating leases. There was no impact to the consolidated statements of operations. Upon adoption, the amount of the ROU assets recorded was offset by the applicable unamortized lease incentive and straight-line lease liability balances of $9.7 million and, therefore, there was no impact to accumulated deficit. There were no initial direct costs related to the leases to consider. The Company’s operating lease liabilities related to its real estate lease agreements were calculated using a collateralized incremental borrowing rate. The weighted average discount rate for the operating lease liability is approximately 11%. A 100-basis point change in the incremental borrowing rate would result in less than a $1 million impact to the ROU assets and liabilities recorded. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term, which for the years ended December 31, 2020, 2019, and 2018 was $4.0 million, $4.3 million, and $5.8 million, respectively, and is included in operating expenses in the consolidated income statements. During 2019, the Company recorded $0.6 million of impairment charges related to its 930 Winter Street lease, which represents the remaining balance of the right to use asset as the likelihood of finding a sub-lessor has diminished significantly as the lease approaches termination. Cash paid against operating lease liabilities during the years ended December 31, 2020 and 2019 was $5.5 million and $5.3 million, respectively. As of December 31, 2020, the Company’s ROU assets and lease liabilities for operating leases totaled $14.1 million and $21.8 million, respectively, and the weighted average remaining term of the operating leases is approximately five years . The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2021 $ 5,323 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 2025 ​ 5,490 Thereafter ​ 1,376 Total lease payments ​ ​ 28,558 Less imputed interest ​ ​ (6,761) Total lease liabilities ​ $ 21,797 ​ In addition to the amounts in the table above, the Company is also responsible for variable operating costs and real estate taxes approximating $3.1 million per year through March 2026. Sublease Income In 2020, the Company executed four agreements to sublease a total of approximately 65,000 square feet of the Company’s leased space at 830 Winter Street, Waltham, Massachusetts through March 2026. During the year ended December 31, 2020, the Company recorded $2.8 million of sublease income, inclusive of the sublessees’ proportionate share of operating expenses and real estate taxes for the period. Two of the four sublease agreements include an early termination option after certain periods of time for an agreed-upon fee. Assuming no early termination option is exercised, the Company will receive $15.9 million in minimum rental payments over the remaining term of the subleases, which is not included in the operating lease liability table above. The sublessees are also responsible for their proportionate share of variable operating expenses and real estate taxes.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies
Commitments and Contingencies	K. Commitments and Contingencies Manufacturing Commitments As of December 31, 2020, the Company has noncancelable obligations under several agreements related to in-process and future manufacturing of antibody and cytotoxic agents required for supply of the Company’s product candidates totaling $6.5 million, which will be paid in 2021. Additionally, pursuant to commercial agreements for future production of antibody, our noncancelable commitments total approximately $36.0 million at December 31, 2020. Litigation The Company is not party to any material litigation.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2020
Employee Benefit Plans
Employee Benefit Plans	L. Employee Benefit Plans The Company has a deferred compensation plan under Section 401(k) of the Internal Revenue Code (the 401(k) Plan). Under the 401(k) Plan, eligible employees are permitted to contribute, subject to certain limitations, up to 100% of their gross salary and the Company’s matching contribution is 50% of the first 6% of the eligible employees’ contributions. In the years ended December 31, 2020, 2019 and 2018, the Company’s contributions to the 401(k) Plan totaled $0.4 million, $0.8 million, and $1.0 million, respectively.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, ImmunoGen Securities Corp., ImmunoGen Europe Limited, ImmunoGen BioPharma (Ireland) Limited, and Hurricane, LLC. All intercompany transactions and balances have been eliminated.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States (U.S.) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Subsequent Events	Subsequent Events The Company has evaluated all events or transactions that occurred after December 31, 2020 up through the date the Company issued these financial statements. Pursuant to an Open Market Sale Agreement SM under which the Company may issue and sell shares of its common stock, from time to time for an aggregate sales price of up to $150.0 million, subsequent to December 31, 2020 and through the date the Company issued these financial statements, the Company has sold 4,544,424 shares of its common stock generating net proceeds of $33.6 million after deducting offering commissions and expenses. The Company did not have any other material recognized or unrecognized subsequent events.
Adoption of ASC Topic 842, Leases	Adoption of ASC 842, Leases The Company adopted Accounting Standards Update (ASU) No. 2016-2, Leases (Topic 842) Leases (Topic 842): Targeted Improvements the previous lease guidance of ASC 840, Leases
Revenue Recognition	Revenue Recognition The Company enters into licensing and development agreements with collaborators for the development of ADCs. The terms of these agreements contain multiple promised goods and services which may include (i) licenses, or options to obtain licenses, to the Company’s ADC technology, (ii) rights to future technological improvements, (iii) technology transfer services and other activities to be performed on behalf of the collaborative partner, and (iv) delivery of cytotoxic agents and/or the manufacture of preclinical and clinical materials for the collaborative partner. Payments to the Company under these agreements may include upfront fees, option fees, exercise fees, payments for services, payments for preclinical or clinical materials, payments based upon the achievement of certain milestones, and royalties on product sales. The Company follows the provisions of ASC 606, Revenue from Contracts with Customers, Revenue is recognized when a 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. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when or as the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations based on its assessment of whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when or as the performance obligation is satisfied. The Company exercises judgment in assessing those promised goods and services that are distinct and thus representative of performance obligations. To the extent the Company identifies multiple performance obligations in a contract, the Company must develop assumptions that require judgment to determine the estimated standalone selling price for each performance obligation in order to allocate the transaction price among the identified performance obligations. These judgments and assumptions are discussed in further detail below. At December 31, 2020, the Company had the following types of material agreements with the parties identified below: ● Development and commercialization licenses, which provide the counterparty with the right to use the Company’s ADC technology and/or certain other intellectual property to develop and commercialize compounds to a specified antigen target: Bayer (one exclusive single-target license) CytomX (two exclusive single-target licenses) Debiopharm (one exclusive single-compound license) Fusion Pharmaceuticals (one exclusive single-target license) Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (one territory-specific exclusive single-compound license) Novartis (five exclusive single-target licenses) Oxford BioTherapeutics/Menarini (one exclusive single-target license sublicensed from Amgen) Roche, through its Genentech unit (five exclusive single-target licenses) Viridian (one exclusive single-target license) ● Collaboration and license agreement to co-develop and co-commercialize a specified anticancer compound on established terms: MacroGenics During the year ended December 31, 2020, pursuant to notices received, the exclusive development and commercialization licenses granted to each of Biotest and Takeda and the collaboration and option agreement with Jazz were terminated. There are no performance, cancellation, termination, or refund provisions in any of the arrangements that contain material financial consequences to the Company. Development and Commercialization Licenses The obligations under a development and commercialization license agreement generally include the license to the Company’s ADC technology with respect to a specified antigen target or compound, and may also include obligations related to rights to future technological improvements and other activities to be performed on behalf of the collaborative partner. Generally, development and commercialization licenses contain non-refundable terms for payments and, depending on the terms of the agreement, provide that the Company will earn payments upon the achievement of certain milestones and royalty payments, generally until the later of the last applicable patent expiration or a fixed period of years after product launch. Royalty rates may vary over the royalty term depending on the Company’s intellectual property rights and/or the presence of comparable competing products. In the case of Debiopharm, no royalties will be received. In certain instances, the Company may also provide cytotoxic agents and/or clinical materials or other services in addition to the development and commercialization licenses. For example, the Company may provide technology transfer services in connection with the out-licensing of product candidates initially developed by the Company, and may also provide technical assistance and share any technology improvements with its collaborators during the term of the collaboration agreements. The Company does not directly control when or whether any collaborator will request certain services, achieve milestones, or become liable for royalty payments. In determining the performance obligations for these arrangements, management evaluates whether the license is distinct and has significant standalone functionality either alone or with other readily available resources based on the consideration of the relevant facts and circumstances for each arrangement. Factors considered in this determination include the research capabilities of the partner and the availability of ADC technology research expertise and ADC manufacturing capabilities in the general marketplace and whether technological improvements are required for the continued functionality of the license. If the license to the Company’s intellectual property is determined to be distinct, the Company recognizes revenues 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. If the license is not distinct, the license is combined with other goods or services into a single performance obligation and revenue is recognized over time. The Company estimates the standalone selling prices of the license and all other performance obligations based on market conditions, similar arrangements entered into by third parties, and entity-specific factors such as the terms of the Company’s previous collaborative agreements, recent preclinical and clinical testing results of therapeutic products that use the Company’s ADC technology, the Company’s pricing practices and pricing objectives, the likelihood that technological improvements will be made, and, if made, will be used by the Company’s collaborators, and the nature of the other services to be performed on behalf of its collaborators and market rates for similar services. The Company recognizes revenue related to technology transfer activities and other services as the services are performed. The Company is generally compensated for these activities at negotiated rates that are consistent with what other third parties would charge. The Company records amounts recognized for research materials provided or services performed as a component of research and development support revenue. The Company may also provide cytotoxic agents and/or preclinical and clinical materials (drug substance/drug product) to its collaborators at negotiated prices generally consistent with what other third parties would charge. The Company recognizes revenue on cytotoxic agents and on preclinical and clinical materials when control transfers to the collaborator. The Company recognizes revenue related to the rights to future technological improvements over the estimated term of the applicable license. The Company’s development and commercialization license agreements have milestone payments which for reporting purposes are aggregated into two categories: (i) development and regulatory milestones, and (ii) sales milestones. Development milestones are typically payable when a product candidate initiates or advances into different clinical trial phases. Regulatory milestones are typically payable upon submission for marketing approval with the FDA or other countries’ regulatory authorities or on receipt of actual marketing approvals for the compound or for additional indications. Sales milestones are typically payable when annual sales reach certain levels. At the inception of each arrangement, the Company evaluates any development and regulatory milestone payments to determine whether the milestone is considered probable of being reached 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 milestone value is included in the transaction price to be allocated; otherwise, such amounts are considered constrained and excluded from the transaction price. As part of its evaluation of the constraint, the Company considers numerous factors, including whether the achievement of the milestone is outside the control of the Company and contingent upon the future success of clinical trials, the collaborator’s efforts, or the receipt of regulatory approval. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development or regulatory milestones and any related constraint, and if necessary, adjusts the estimate of the transaction price. In addition, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. Amounts allocated to a satisfied performance obligation are recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. For development and commercialization license agreements that include sales-based royalties, including milestone payments based on the level of sales, 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) in accordance with the royalty recognition constraint. Under the Company’s development and commercialization license agreements, except for the Debiopharm license, the Company receives royalty payments based upon its licensees’ net sales of covered products. Generally, under the development and commercialization agreements, the Company receives royalty reports and payments from its licensees approximately one quarter in arrears. The Company estimates the amount of royalty revenue to be recognized based on historical and forecasted sales and/or sales information from its licensees if available. Collaboration and Option Agreements/Right-to-Test Agreements The Company’s right-to-test agreements provide collaborators the right to test the Company’s ADC technology for a defined period of time through a research, or right-to-test, license. Under both right-to-test agreements and collaboration and option agreements, collaborators may (a) “take” options, for a defined period of time, to specified targets and (b) upon exercise of those options, secure or “take” licenses to develop and commercialize products for the specified targets on established terms. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement (referred to as “upfront” fees or payments), (ii) upon the opt-in to acquire a development and commercialization license(s) (referred to as exercise fees or payments earned, if any, when the development and commercialization license is “taken”), (iii) after providing services at the collaborator’s request at negotiated prices, which are generally consistent with what other third parties would charge, or (iv) upon some combination of all of these fees. The accounting for collaboration and option agreements and right-to-test agreements is dependent on the nature of the options granted to the collaborative partner. Options are considered distinct performance obligations if they provide a collaborator with a material right. Factors that are considered in evaluating whether options convey a material right include the overall objective of the arrangement, the benefit the collaborator might obtain from the agreement without exercising the options, the cost to exercise the options relative to the fair value of the licenses, and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. As of December 31, 2020, all option and right-to-test agreements have expired or terminated. If the Company concludes that an option provides the customer a material right, and therefore is a separate performance obligation, the Company then determines the estimated standalone selling price of the option using the following inputs: (a) estimated fair value of the license underlying each option, (b) the amount the partner would pay to exercise the option to obtain the license, and (c) probability of exercise. The Company does not control when or if any collaborator will exercise its options for development and commercialization licenses. As a result, the Company cannot predict when or if it will recognize revenues in connection with any of the foregoing. Upfront payments on development and commercialization licenses may be recognized upon delivery of the license if facts and circumstances dictate that the license is distinct from the other promised goods and services. In determining whether a collaboration and option agreement is within the scope of ASC 808, Collaborative Arrangements Transaction Price Allocated to Remaining Performance Obligations Deferred revenue under ASC 606 represents the portion of the transaction price received under various contracts for which work has not been performed (or has been partially performed) and includes unexercised contract options that are considered material rights. As of December 31, 2020, the aggregate amount of the transaction price allocated to remaining performance obligations comprising deferred revenue was $110.1 million. The Company expects to recognize revenue on approximately 27%, 65%, and 8% of the remaining performance obligations over the next 12 months , 13 to 60 months , and 61 to 120 months , respectively, however, it does not control when or if any collaborator will terminate existing development and commercialization licenses. Contract Balances from Contracts with Customers The following tables present changes in the Company’s contract assets and contract liabilities during the years ended December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Year ended December 31, 2020 ​ December 31, 2019 Additions ​ Deductions ​ Impact of Netting ​ December 31, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ 42,050 ​ $ (63,742) ​ $ 4,369 ​ $ 110,109 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Year ended December 31, 2019 ​ December 31, 2018 ​ Additions ​ Deductions ​ Impact of Netting ​ December 31, 2019 Contract asset ​ $ 500 ​ $ 8,000 ​ $ (500) ​ $ (4,369) ​ $ 3,631 Contract liabilities (deferred revenue) ​ $ 80,802 ​ $ 65,816 ​ $ (14,817) ​ $ (4,369) ​ $ 127,432 ​ During the years ended December 31, 2020, 2019, and 2018 the Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ ​ December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 61,872 ​ $ 14,817 ​ $ 14,139 ​ Performance obligations satisfied in previous periods ​ $ — ​ $ 12,672 ​ $ 1,476 ​ ​ During 2020, the Company recognized $60.5 million of previously deferred license revenue upon Jazz’s opt-out of its right to the last remaining license under the agreement and $3.2 million of upfront fees previously received from other partners, of which $1.4 million was included in contract liabilities at the beginning of 2020. A $40.0 million upfront payment received in 2020 pursuant to a license agreement executed with Huadong was recorded as deferred revenue and none of this amount was recognized as revenue during 2020. Additionally, a contract asset of $3.6 million, net of $4.4 million in related contract liabilities, was recorded for two probable milestones in 2019 pursuant to license agreements with CytomX and Novartis, which were subsequently achieved and paid during 2020. The Company recorded the following during the year ended December 31, 2019: (i) license and milestone fee revenue of $7.7 million for probable development milestones pursuant to license agreements with CytomX and Novartis, with another $0.3 million deferred which represents the amount allocated to future rights to technological improvements; a $3.6 million contract asset was recorded in December 2019 related to these probable milestones, net of a $4.4 million reduction in related contract liabilities; (ii) a $5 million regulatory milestone payment earned under its license agreement with Genentech, a member of the Roche Group; the full amount of the milestone was recognized as revenue in the period as the amount allocated to future rights to technological improvements was not material; (iii) $14.5 million of previously deferred license revenue recognized upon the opt-out of the right to execute a license by Jazz; (iv) $65.2 million was recorded as deferred revenue as a result of a sale of the Company’s residual rights to receive royalty payments on commercial sales of Kadcyla ® As a result of adoption of ASC 606, a contract asset of $5.0 million was recorded for a probable milestone which was subsequently earned and paid during the year ended December 31, 2018. Additionally the Company recorded the following during 2018: (i) a contract asset and related revenue of $0.5 million for a probable milestone pursuant to a license agreement with Fusion Pharmaceuticals, which was subsequently paid in 2019; (ii) a $1 million development milestone earned under a sublicense agreement with Oxford BioTherapeutics Ltd. as license and milestone fee revenue, which was included in accounts receivable as of December 31, 2018; (iii) $10.9 million of revenue previously deferred, with a net reduction in deferred revenue of $5.9 million due to contract asset and contract liability netting as a result of Takeda not executing a second license it had available, or extending or expanding its right-to-test agreement; (iv) $0.8 million of revenue previously deferred upon completion of Debiopharm and another collaborator’s performance obligations; (v) $2.1 million of revenue previously deferred related to numerous collaborators’ rights to technological improvements; and (vi) $0.3 million of revenue previously deferred upon shipment of clinical materials to a partner which is included in clinical material revenue. The timing of revenue recognition, billings, and cash collections results in billed receivables, unbilled receivables, contract assets, and contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded (under the caption deferred revenue). Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met.
Financial Instruments and Concentration of Credit Risk	Financial Instruments and Concentration of Credit Risk Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. Deposits with banks may exceed the amount of insurance provided on such deposits. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal risk. The Company’s cash equivalents consist of money market funds with underlying investments primarily being U.S. Government-issued securities and high quality, short-term commercial paper. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents, and marketable securities. The Company held no marketable securities as of December 31, 2020 or 2019. The Company’s investment policy, approved by the Board of Directors, limits the amount it may invest in any one type of investment, thereby reducing credit risk concentrations.
Cash and Cash Equivalents	Cash and Cash Equivalents All highly liquid financial instruments with maturities of three months or less when purchased are considered cash equivalents. As of December 31, 2020 and 2019, the Company held $293.9 million and $176.2 million, respectively, in cash and money market funds consisting principally of U.S. Government-issued securities and high quality, short-term commercial paper which were classified as cash and cash equivalents.
Non-cash Investing	Non-cash Investing Activities The Company had $0.7 million of accrued capital expenditures as of December 31, 2020 which have been treated as a non-cash investing activity and, accordingly, not reflected in the consolidated statement of cash flows. The Company had no accrued capital expenditures as of December 31, 2019.
Fair Value of Financial Instruments	Fair Value of Financial Instruments ASC 820, Fair Value Measurement ● Level 1—Quoted prices in active markets for identical assets or liabilities. ● Level 2—Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of December 31, 2020 and 2019, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following tables represent the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of each date (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 194,525 ​ $ 194,525 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 163,674 ​ $ 163,674 $ — $ — ​ The fair value of the Company’s cash equivalents is based primarily on quoted prices from active markets. The carrying amounts reflected in the consolidated balance sheets for accounts receivable, unbilled receivable, contract assets, non-cash royalty receivable, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature.
Unbilled Revenue/Reimbursement	Unbilled receivable primarily represents research funding earned based on actual resources utilized and external expenses incurred under certain of the Company’s collaborator agreements.
Clinical Trial Accruals	Clinical Trial Accruals Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these activities to third parties. Third-party clinical trial expenses include investigator fees, site costs (patient cost), clinical research organization costs, and costs for central laboratory testing and data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through cost. These inputs are required to be estimated due to a lag in receiving the actual clinical information from third parties. . Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed.
Leases	Leases Effective January 1, 2019, the Company adopted ASU 2016-2, the details of which are further discussed in Note J. The Company determines if an arrangement is a lease at inception. Operating leases include right-of-use (ROU) assets and operating lease liabilities (current and non-current), which are recorded in the Company’s consolidated balance sheets. Single payment capital leases for equipment that are considered finance leases are included in property and equipment in the Company’s consolidated balance sheets. As the single payment obligations have all been made, there is no related liability recorded. ROU 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. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses the implicit rate when readily determinable. As a number of the Company’s leases do not provide an implicit rate, the Company uses an incremental borrowing rate applicable to the Company based on the information available at the commencement date in determining the present value of lease payments. As the Company has no existing or proposed collateralized borrowing arrangements, to determine a reasonable incremental borrowing rate, the Company considers collateral assumptions, the lease term, the Company’s current credit risk profile, and rates for existing borrowing arrangements for comparable peer companies. The operating lease ROU assets were netted against any lease incentive and straight-line lease liability balances at January 1, 2019 upon adoption of ASC 842. The Company accounts for the lease and fixed non-lease components as a single lease component. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term.
Other Accrued Liabilities	Other Accrued Liabilities Other accrued liabilities consisted of the following at December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, December 31, ​ 2020 ​ 2019 Accrued contract payments ​ $ 15,576 ​ $ 5,188 Accrued clinical trial costs ​ 11,401 ​ 6,418 Accrued professional services ​ 1,200 ​ 1,274 Accrued employee benefits ​ 39 ​ 314 Accrued public reporting charges ​ 319 ​ 180 Other current accrued liabilities ​ 785 ​ 558 Total ​ $ 29,320 ​ $ 13,932 ​
Research and Development Expenses	The Company’s research and development expenses are charged to expense as incurred and relate to (i) research to evaluate new targets and to develop and evaluate new antibodies, linkers, and cytotoxic agents, (ii) preclinical testing of its own and, in certain instances, its collaborators’ product candidates, and the cost of its own clinical trials, (iii) development related to clinical and commercial manufacturing processes, and (iv) external manufacturing operations and, prior to 2019, internal manufacturing operations, which also included raw materials. Payments made by the Company in advance for research and development services not yet provided and/or materials not yet delivered and accepted are recorded as prepaid expenses and are included in the accompanying consolidated balance sheets as prepaid and other current assets.
Income Taxes	Income Taxes The Company uses the liability method to account for income taxes. Deferred tax assets and liabilities are determined based on differences between the financial reporting and income tax basis of assets and liabilities, as well as net operating loss carry forwards and tax credits and are measured using the enacted tax rates and laws that will be in effect when the differences reverse. A valuation allowance against net deferred tax assets is recorded if, based on the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.
Property and Equipment	Property and Equipment Property and equipment are stated at cost. The Company provides for depreciation based upon expected useful lives using the straight-line method over the following estimated useful lives: ​ ​ ​ ​ ​ Machinery and equipment 5 years ​ Computer hardware and software 3 years ​ Furniture and fixtures 5 years ​ Leasehold improvements Shorter of remaining lease term or 7 years ​ ​ Equipment under capital leases is amortized over the lives of the respective leases or the estimated useful lives of the assets, whichever is shorter, and included in depreciation expense. Maintenance and repairs are charged to expense as incurred. Upon retirement or sale, the cost of disposed assets and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the statement of operations. The Company recorded net gains (losses) of $0.7 million, $(1.7) million, and $(0.1) million related to impairment charges and the sale/disposal of certain furniture and equipment during the years ended December 31, 2020, 2019, and 2018, respectively.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company continually evaluates whether events or circumstances have occurred that indicate that the estimated remaining useful life of its long-lived assets may warrant revision or that the carrying value of these assets may be impaired if impairment indicators are present. The Company evaluates the realizability of its long-lived assets based on cash flow expectations for the related asset. Any write-downs to fair value are treated as permanent reductions in the carrying amount of the assets. Accordingly, during the year ended December 31, 2019, the Company recorded a $2.5 million asset impairment charge resulting from restructuring activities, the details of which are further discussed in Note I. Based on this evaluation, except for the impairment recognized during 2019, the Company believes that none of the Company’s remaining long-lived assets were impaired.
Computation of Net Loss per Common Share	Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted average number of common shares outstanding during the period. During periods of income, participating securities are allocated 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 (the “two-class method”). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted (loss) income per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for the options and unvested restricted stock and the if-converted method for the convertible notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, ​ 2020 ​ 2019 ​ 2018 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 20,873 ​ 14,815 ​ 17,380 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock 1,301 ​ 1,020 ​ 3,001 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 ​ 501 Common stock equivalents under if-converted method for convertible notes ​ 501 ​ 501 ​ 501 ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position.
Stock-Based Compensation	Stock-based Compensation As of December 31, 2020, the Company is authorized to grant future awards under three employee share-based compensation plans, which are the ImmunoGen, Inc. 2018 Employee, Director and Consultant Equity Incentive Plan, or the 2018 Plan, the Employee Stock Purchase Plan, or ESPP, and the ImmunoGen Inducement Equity Incentive Plan, or the Inducement Plan. At the annual meeting of shareholders on June 20, 2018, the 2018 Plan was approved and provides for the issuance of Stock Grants, the grant of Options, and the grant of Stock-Based Awards for up to 7,500,000 shares of the Company’s common stock, as well as up to 19,500,000 shares of common stock which represent awards granted under the previous stock option plans, the ImmunoGen, Inc. 2016 and 2006 Employee, Director and Consultant Equity Incentive Plans, or the 2016 and 2006 Plans, that forfeit, expire, or cancel without delivery of shares of common stock or which resulted in the forfeiture of shares of common stock back to the Company subsequent to June 19, 2018. The Inducement Plan was approved the by Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 1,500,000 shares of the Company’s common stock. Options awarded under the two plans are granted with an exercise price equal to the market price of the Company’s stock at the date of grant. Options vest at various periods of up to four years and may be exercised within ten years of the date of grant. The stock-based awards are accounted for under ASC 718, “Compensation—Stock Compensation.” Pursuant to ASC 718, the estimated grant date fair value of awards is charged to the statement of operations over the requisite service period, which is the vesting period. The fair value of each stock option is estimated on the date of grant using the Black- Scholes option-pricing model with the weighted average assumptions noted in the following table. As the Company has not paid dividends since inception, nor does it expect to pay any dividends for the foreseeable future, the expected dividend yield assumption is zero. Expected volatility is based exclusively on historical volatility of the Company’s stock. The expected term of stock options granted is based exclusively on historical data and represents the period of time that stock options granted are expected to be outstanding. The expected term is calculated for and applied to one group of stock options as the Company does not expect substantially different exercise or post-vesting termination behavior amongst its employee population. The risk-free rate of the stock options is based on the U.S. Treasury rate in effect at the time of grant for the expected term of the stock options. ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 ​ 2019 ​ 2018 Dividend None ​ None ​ None Volatility 85.07% ​ 76.67% ​ 71.02% Risk-free interest rate 1.21% ​ 2.20% ​ 2.73% Expected life (years) 6.0 ​ 6.0 ​ 6.0 ​ Using the Black-Scholes option-pricing model, the weighted average grant date fair values of options granted during the years ended December 31, 2020, 2019, and 2018, were $3.28, $2.81, and $6.70 per share, respectively. A summary of option activity under the option plans for 2020 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- Weighted- ​ ​ ​ ​ Number ​ Average ​ Average ​ Aggregate ​ ​ of Stock ​ Exercise ​ Remaining ​ Intrinsic ​ ​ Options ​ Price ​ Life in Yrs. ​ Value Outstanding at December 31, 2019 ​ 13,518 ​ $ 7.53 ​ ​ ​ ​ ​ Granted ​ 7,421 ​ ​ 4.60 ​ ​ ​ ​ ​ Exercised ​ (336) ​ ​ 2.94 ​ ​ ​ ​ ​ Forfeited/Canceled ​ (2,205) ​ ​ 10.28 ​ ​ ​ ​ ​ Outstanding at December 31, 2020 ​ 18,398 ​ ​ 6.10 ​ 7.66 ​ $ 31,110 Outstanding at December 31, 2020—vested or unvested and expected to vest 17,830 ​ $ 6.15 ​ 7.61 ​ $ 29,990 Exercisable at December 31, 2020 6,983 ​ $ 8.15 ​ 5.82 ​ $ 9,222 ​ In September 2018, the Company granted 295,200 performance-based stock options to certain employees that will vest in two equal installments upon the achievement of specified performance goals. At December 31, 2020, 128,700 of these options are still outstanding. In the year ended December 31, 2020, the Company issued 2.6 million additional performance stock options that will vest in four installments upon the achievement of specified performance goals. The Company determined it is not currently probable that any of these performance goals will be achieved and, therefore, no expense has been recorded to date. The fair value of the performance-based options that could be expensed in future periods is $9.4 million. A summary of restricted stock and restricted stock unit activity under the option plans as for 2020 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2019 1,297 ​ $ 2.97 Vested (749) ​ ​ 2.60 Forfeited ​ (487) ​ ​ 3.62 Unvested at December 31, 2020 ​ 61 ​ $ 2.47 In 2016, 2017, and 2019, the Company granted shares of performance-based restricted common stock to certain employees of the Company. All but 57,400 of these granted shares have since been forfeited. The restrictions on these shares will lapse in three equal installments upon the achievement of specified performance goals. The Company determined it is not currently probable that these performance goals will be achieved and, therefore, no expense has been recorded to date. The fair value of the performance-based shares that could be expensed in future periods is $0.1 million. In June 2018, the Company's Board of Directors, with shareholder approval, adopted the Employee Stock Purchase Plan. Following the automatic share increase on January 1, 2021 under the ESPP’s “evergreen” provision, an aggregate of 2,000,000 shares of common stock have been reserved for issuance under the ESPP. Under the ESPP, eligible participants purchase shares of the Company's common stock at a price equal to 85% of the lesser of the closing price of the Company's common stock on the first business day and the final business day of the applicable plan purchase period. Plan purchase periods are six months and begin on January 1 and July 1 of each year, with purchase dates occurring on the final business day of the given purchase period. The fair value of each ESPP award is estimated on the first day of the offering period using the Black-Scholes option-pricing model. The Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period. During 2020 and 2019, approximately 122,000 and 356,000 shares, respectively, were issued to participating employees at fair values ranging from $1.20 to $2.14 per share. Stock compensation expense related to stock options and restricted stock awards granted under the option plans and the ESPP was $14.0 million, $13.8 million, and $16.4 million during the years ended December 31, 2020, 2019 and 2018, respectively. As of December 31, 2020, the estimated fair value of unvested employee awards was $17.8 million. The weighted-average remaining vesting period for these awards is approximately 2.6 years. Also included in stock and deferred stock unit compensation expense in the consolidated statements of cash flows for the years ended December 31, 2020, 2019, and 2018 is $0.4 million, $0.3 million, and $0.4 million, respectively, of expense recorded for directors’ deferred share units, the details of which are discussed in Note H. A summary of option activity for options vested during the years ended December 31, 2020, 2019, and 2018 is presented below (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 Total fair value of options vested ​ $ 11,465 ​ $ 13,747 ​ $ 7,496 ​ Total intrinsic value of options exercised ​ ​ 746 ​ 556 ​ 3,787 ​ Cash received for exercise of stock options ​ ​ 1,471 ​ 2,873 ​ 4,301 ​ ​
Comprehensive Loss	Comprehensive Loss The Company presents comprehensive loss in accordance with ASC 220, Comprehensive Income.
Segment Information	Segment Information During all periods presented, the Company continued to operate in one reportable business segment under the management approach of ASC 280, Segment Reporting The percentages of revenues recognized from significant customers of the Company in the years ended December 31, 2020, 2019, and 2018 are included in the following table: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, Collaborative Partner: ​ 2020 ​ 2019 ​ 2018 CytomX ​ -% ​ 13% ​ 8% Roche ​ 53% ​ 64% ​ 60% Takeda ​ 1% ​ -% ​ 23% Jazz ​ 46% ​ 18% ​ -% ​ There were no other customers of the Company with significant revenues in the periods presented.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments Recently issued accounting pronouncements, not yet adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes No other recently issued or effective ASUs had, or are expected to have, a material effect on the Company's results of operations, financial condition, or liquidity.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Contract assets and contract liabilities	The following tables present changes in the Company’s contract assets and contract liabilities during the years ended December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Year ended December 31, 2020 ​ December 31, 2019 Additions ​ Deductions ​ Impact of Netting ​ December 31, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ 42,050 ​ $ (63,742) ​ $ 4,369 ​ $ 110,109 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Year ended December 31, 2019 ​ December 31, 2018 ​ Additions ​ Deductions ​ Impact of Netting ​ December 31, 2019 Contract asset ​ $ 500 ​ $ 8,000 ​ $ (500) ​ $ (4,369) ​ $ 3,631 Contract liabilities (deferred revenue) ​ $ 80,802 ​ $ 65,816 ​ $ (14,817) ​ $ (4,369) ​ $ 127,432 ​ During the years ended December 31, 2020, 2019, and 2018 the Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ ​ December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 61,872 ​ $ 14,817 ​ $ 14,139 ​ Performance obligations satisfied in previous periods ​ $ — ​ $ 12,672 ​ $ 1,476 ​
Schedule of assets that are required to be measured at fair value on a recurring basis	As of December 31, 2020 and 2019, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following tables represent the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of each date (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 194,525 ​ $ 194,525 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 163,674 ​ $ 163,674 $ — $ —
Schedule of components of other accrued liabilities	Other accrued liabilities consisted of the following at December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, December 31, ​ 2020 ​ 2019 Accrued contract payments ​ $ 15,576 ​ $ 5,188 Accrued clinical trial costs ​ 11,401 ​ 6,418 Accrued professional services ​ 1,200 ​ 1,274 Accrued employee benefits ​ 39 ​ 314 Accrued public reporting charges ​ 319 ​ 180 Other current accrued liabilities ​ 785 ​ 558 Total ​ $ 29,320 ​ $ 13,932
Schedule of estimated useful lives of property and equipment	​ ​ ​ ​ ​ Machinery and equipment 5 years ​ Computer hardware and software 3 years ​ Furniture and fixtures 5 years ​ Leasehold improvements Shorter of remaining lease term or 7 years ​
Schedule of common stock equivalents, as calculated in accordance with the treasury-stock method	The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for the options and unvested restricted stock and the if-converted method for the convertible notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, ​ 2020 ​ 2019 ​ 2018 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 20,873 ​ 14,815 ​ 17,380 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock 1,301 ​ 1,020 ​ 3,001 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 ​ 501 Common stock equivalents under if-converted method for convertible notes ​ 501 ​ 501 ​ 501
Schedule of assumptions	​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 ​ 2019 ​ 2018 Dividend None ​ None ​ None Volatility 85.07% ​ 76.67% ​ 71.02% Risk-free interest rate 1.21% ​ 2.20% ​ 2.73% Expected life (years) 6.0 ​ 6.0 ​ 6.0
Summary of stock option activity	A summary of option activity under the option plans for 2020 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- Weighted- ​ ​ ​ ​ Number ​ Average ​ Average ​ Aggregate ​ ​ of Stock ​ Exercise ​ Remaining ​ Intrinsic ​ ​ Options ​ Price ​ Life in Yrs. ​ Value Outstanding at December 31, 2019 ​ 13,518 ​ $ 7.53 ​ ​ ​ ​ ​ Granted ​ 7,421 ​ ​ 4.60 ​ ​ ​ ​ ​ Exercised ​ (336) ​ ​ 2.94 ​ ​ ​ ​ ​ Forfeited/Canceled ​ (2,205) ​ ​ 10.28 ​ ​ ​ ​ ​ Outstanding at December 31, 2020 ​ 18,398 ​ ​ 6.10 ​ 7.66 ​ $ 31,110 Outstanding at December 31, 2020—vested or unvested and expected to vest 17,830 ​ $ 6.15 ​ 7.61 ​ $ 29,990 Exercisable at December 31, 2020 6,983 ​ $ 8.15 ​ 5.82 ​ $ 9,222
Summary of restricted stock activity	​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2019 1,297 ​ $ 2.97 Vested (749) ​ ​ 2.60 Forfeited ​ (487) ​ ​ 3.62 Unvested at December 31, 2020 ​ 61 ​ $ 2.47
Summary of vested stock option activity	A summary of option activity for options vested during the years ended December 31, 2020, 2019, and 2018 is presented below (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 Total fair value of options vested ​ $ 11,465 ​ $ 13,747 ​ $ 7,496 ​ Total intrinsic value of options exercised ​ ​ 746 ​ 556 ​ 3,787 ​ Cash received for exercise of stock options ​ ​ 1,471 ​ 2,873 ​ 4,301 ​ ​
Schedule of percentage of total revenues recognized from each significant customer	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, Collaborative Partner: ​ 2020 ​ 2019 ​ 2018 CytomX ​ -% ​ 13% ​ 8% Roche ​ 53% ​ 64% ​ 60% Takeda ​ 1% ​ -% ​ 23% Jazz ​ 46% ​ 18% ​ -%

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property and Equipment
Schedule of components of property and equipment	Property and equipment consisted of the following at December 31, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2020 ​ 2019 ​ Leasehold improvements ​ $ 21,890 $ 20,776 ​ Machinery and equipment ​ 2,861 ​ 9,384 ​ Computer hardware and software ​ 5,636 ​ 5,692 ​ Furniture and fixtures ​ 3,039 ​ 3,607 ​ Assets under construction ​ 97 ​ — ​ ​ ​ $ 33,523 ​ $ 39,459 ​ Less accumulated depreciation ​ (27,763) ​ (32,466) ​ Property and equipment, net ​ $ 5,760 ​ $ 6,993 ​

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2020
Liability Related to Sale of Future Royalties
Schedule of Liability account during the period from the inception of the royalty transaction	The following table shows the activity within the liability account during the year ended December 31, 2020 and the period from inception (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Period from ​ ​ Year Ended ​ inception to ​ December 31, 2020 ​ December 31, 2020 Liability related to sale of future royalties, net — beginning balance ​ $ 123,541 ​ $ — Proceeds from sale of future royalties, net ​ — ​ 194,135 Kadcyla royalty payments received and paid ​ (61,195) ​ (206,367) Non-cash interest expense recognized ​ ​ 23,093 ​ ​ 97,671 Liability related to sale of future royalties, net — ending balance ​ $ 85,439 ​ $ 85,439

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Taxes
Reconciliation of the Company's expected tax benefit, as computed by applying the U.S. federal corporate tax rate to loss before the benefit for income taxes, and actual tax	The difference between the Company’s expected tax benefit, as computed by applying the applicable U.S. federal corporate tax rate to loss before the benefit for income taxes, and actual tax is reconciled in the following chart (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Years Ended December 31, ​ ​ 2020 2019 ​ 2018 Loss before income tax expense $ (44,372) $ (104,133) $ (168,843) Expected tax benefit at 21% $ (9,318) ​ $ (21,868) ​ $ (35,457) Permanent differences ​ 157 ​ 320 ​ (103) Incentive stock options ​ ​ 201 ​ ​ 569 ​ ​ 1,144 State tax benefit net of federal benefit ​ (2,250) ​ (6,726) ​ (10,622) Change in valuation allowance, net ​ 15,175 ​ 27,812 ​ 53,706 Federal research credit ​ (228) ​ (1,652) ​ (2,466) Federal orphan drug credit ​ ​ (6,218) ​ ​ (4,426) ​ ​ (6,934) Expired loss and credit carryforwards ​ ​ 419 ​ ​ 500 ​ ​ — Lease incentive ​ ​ — ​ ​ — ​ ​ 109 Stock option expirations ​ 2,062 ​ 5,471 ​ 623 Benefit for income taxes ​ $ — ​ $ — ​ $ —
Schedule of significant components of deferred tax assets	​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 ​ 2019 Deferred tax assets: ​ ​ ​ ​ Net operating loss carryforwards $ 220,286 ​ $ 191,744 Research and development tax credit carryforwards ​ 80,694 ​ 75,084 Property and other intangible assets ​ 871 ​ 809 Deferred revenue ​ 30,082 ​ 36,008 Stock-based compensation ​ 9,940 ​ 9,630 Operating lease liability ​ 6,033 ​ 6,767 Other liabilities ​ 1,514 ​ 2,255 Royalty sale ​ 17,455 ​ ​ 30,030 Total deferred tax assets $ 366,875 ​ $ 352,327 ​ ​ ​ ​ ​ ​ Deferred tax liabilities: ​ ​ ​ ​ ​ Stock-based compensation ​ (58) ​ ​ (110) Operating lease right of use asset ​ (3,844) ​ ​ (4,258) Royalty sale transaction costs ​ (247) ​ ​ (408) Total deferred tax liabilities $ (4,149) ​ $ (4,776) Valuation allowance ​ (362,726) ​ (347,551) Net deferred tax assets/(liabilities) $ — ​ $ —

Capital Stock (Tables)

Capital Stock (Tables)	12 Months Ended
	Dec. 31, 2020
Capital Stock
Schedule of options exercisable and their respective weighted average exercise prices per share	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ‑ ​ ​ Exercisable ​ Average ​ ​ (in thousands) ​ Exercise Price December 31, 2020 6,983 ​ $ 8.15 ​ December 31, 2019 5,801 ​ $ 10.16 ​ December 31, 2018 8,405 ​ $ 11.47 ​

Restructuring Charges (Tables)

Restructuring Charges (Tables)	12 Months Ended
	Dec. 31, 2020
Restructuring Charge
Schedule activity against the restructuring charge related to the employee terminations	​ ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2019 ​ $ 4,087 Additional charges/adjustments during the period ​ ​ (116) Payments during the period ​ ​ (3,187) Balance at December 31, 2020 ​ $ 784

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases
Schedule of maturities of operating lease liabilities	The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2021 $ 5,323 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 2025 ​ 5,490 Thereafter ​ 1,376 Total lease payments ​ ​ 28,558 Less imputed interest ​ ​ (6,761) Total lease liabilities ​ $ 21,797

Nature of Business and Plan o_2

Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	1 Months Ended	2 Months Ended	12 Months Ended
	Jan. 31, 2021	Mar. 01, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net loss			$ (44,372)	$ (104,133)	$ (168,843)
Accumulated deficit			(1,331,840)	(1,287,468)
Total revenues			132,299	82,271	53,446
Cash and cash equivalents			$ 293,856	$ 176,225	$ 262,252	$ 267,107
Number of months Capital resources meets capital expenditures			12 months
Product
Total revenues			$ 0
Open Market Sale Agreement | Subsequent event
Consideration received from sale of common stock	$ 33,600	$ 33,600

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Subsequent Events (Details) - Open Market Sale Agreement - USD ($) $ in Millions	1 Months Ended	2 Months Ended
	Jan. 31, 2021	Mar. 01, 2021
Subsequent Event [Line Items]
Aggregate sales price		$ 150
Number of shares sold		4,544,424
Subsequent event
Subsequent Event [Line Items]
Consideration received from sale of common stock	$ 33.6	$ 33.6

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021item	Dec. 31, 2020USD ($)item	Dec. 31, 2010item
Bayer
Summary of Significant Accounting Policies
Number of single-target licenses		1
CytomX
Summary of Significant Accounting Policies
Number of single-target licenses		2
Debiopharm
Summary of Significant Accounting Policies
Number of single-compound licenses		1
Future milestones or royalties to be received | $		$ 0
Fusion Pharmaceuticals
Summary of Significant Accounting Policies
Number of single-target licenses		1
Lilly
Summary of Significant Accounting Policies
Number of single-target licenses		3
Novartis
Summary of Significant Accounting Policies
Number of single-target licenses		5	6
Oxford BioTherapeutics Ltd Member
Summary of Significant Accounting Policies
Number of single-target licenses		1
Roche
Summary of Significant Accounting Policies
Number of single-target licenses		5
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd
Summary of Significant Accounting Policies
Number of single-compound licenses	1
Viridian
Summary of Significant Accounting Policies
Number of single-target licenses		1

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Performance Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 110.1
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	27.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	65.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	8.00%
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	12 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	13 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction		61 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	60 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction		120 months

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance	$ 3,631	$ 500
Contract asset, Additions		8,000
Contract asset, Deductions	(8,000)	(500)
Contract Asset, Impact Of Netting.	4,369	(4,369)
Contract asset, Ending balance		3,631
Contract liabilities:
Contract liabilities (deferred revenue), Beginning balance	127,432	80,802
Contract liabilities (deferred revenue), Additions	42,050	65,816
Contract liabilities (deferred revenue), Deductions	(63,742)	(14,817)
Contract Liabilities (deferred revenue), Impact Of Netting	4,369	(4,369)
Contract liabilities (deferred revenue), Ending balance	$ 110,109	$ 127,432

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Revenues Recognized as a Result of Changes in Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	$ 61,872	$ 14,817	$ 14,139
Performance obligations satisfied in previous periods		$ 12,672	$ 1,476

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($)	1 Months Ended				12 Months Ended
	Dec. 31, 2019	Mar. 31, 2018	Aug. 31, 2017	Mar. 31, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2016	Dec. 31, 2000
Revenue recognized in the period from:
Revenue from contract with customer					$ 132,299,000	$ 82,271,000	$ 53,446,000
Deferred revenue	$ 127,432,000				110,109,000	127,432,000	80,802,000
Contract assets	3,631,000					3,631,000	500,000
Contract liability	309,000				29,249,000	309,000
Contract Liabilities (deferred revenue), Impact Of Netting	(4,369,000)				4,369,000	(4,369,000)
Revenue recognized, previously deferred					61,872,000	14,817,000	14,139,000
Milestone earned, included in accounts receivable					(8,000,000)	(500,000)
Probable milestone earned and paid						(8,000,000)
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer					63,742,000	34,788,000	15,280,000
Upfront payment
Revenue recognized in the period from:
Contract liability					1,400,000
Total Revenue Recognized From Both The Beginning Balance And Current Period Increase In Contract Liability					3,200,000
Clinical materials revenue
Revenue recognized in the period from:
Revenue from contract with customer							4,635,000
Upon shipment | Clinical materials revenue
Revenue recognized in the period from:
Revenue recognized, previously deferred							300,000
CytomX and Novartis | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer						7,700,000
CytomX and Novartis | Technological Improvements
Revenue recognized in the period from:
Deferred revenue	300,000					300,000
CytomX and Novartis | Probable Milestone
Revenue recognized in the period from:
Contract assets	3,600,000					3,600,000
Contract Liabilities (deferred revenue), Impact Of Netting	(4,400,000)					(4,400,000)
Roche | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer										$ 2,000,000
Genentech | Regulatory milestones
Revenue recognized in the period from:
Revenue from contract with customer						5,000,000
CytomX
Revenue recognized in the period from:
Deferred revenue	200,000					200,000			$ 13,000,000
CytomX | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	7,300,000							$ 12,700,000
Deferred revenue					200,000
CytomX | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer					7,500,000
CytomX | Probable Milestone
Revenue recognized in the period from:
Contract assets					3,600,000
Contract liability					4,400,000
Jazz
Revenue recognized in the period from:
Revenue recognized, previously deferred						14,500,000
Jazz | License and milestone fees
Revenue recognized in the period from:
Amortization of deferred revenue					60,500,000
Jazz | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer			$ 75,000,000
Kadcyla | Royalty revenue
Revenue recognized in the period from:
Deferred revenue	$ 65,200,000					65,200,000
Takeda
Revenue recognized in the period from:
Deferred revenue, Deductions							(5,900,000)
Takeda | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer		$ 10,900,000			900,000				$ 8,600,000
Takeda | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer				$ 20,000,000
Amgen/Oxford BioTherapeutics
Revenue recognized in the period from:
Revenue from contract with customer					4,000,000
Amgen/Oxford BioTherapeutics | License and milestone fees
Revenue recognized in the period from:
Milestone earned, included in accounts receivable							1,000,000
Amgen/Oxford BioTherapeutics | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer										$ 1,000,000
Fusion Pharmaceuticals | Probable Milestone | Milestone related
Revenue recognized in the period from:
Revenue from contract with customer						500,000
Debiopharm and Another Collaborators
Revenue recognized in the period from:
Revenue recognized, previously deferred							800,000
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue						$ 300,000	2,100,000
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd | Upfront payment
Revenue recognized in the period from:
Deferred revenue					40,000,000
Revenue recognized, previously deferred					$ 0
Right-to-test agreement | Takeda
Revenue recognized in the period from:
Revenue recognized, previously deferred							10,900,000
Adjustments due to new guidance | ASU 2014-09
Revenue recognized in the period from:
Revenue from contract with customer							$ 5,000,000

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Financial Instruments and Concentration of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2020USD ($)Institution	Dec. 31, 2019USD ($)
Financial Instruments and Concentration of Credit Risk
Number of financial institutions in the U.S. in which cash and cash equivalents are primarily maintained | Institution	3
Marketable securities held by entity | $	$ 0	$ 0

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Cash and cash equivalents	$ 293,856	$ 176,225	$ 262,252	$ 267,107

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Non-cash Investing (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Summary of Significant Accounting Policies
Accrued capital expenditures	$ 700,000	$ 0

Summary of Significant Accou_13

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%	4.50%
Principal amount of debt for conversion calculations	$ 1,000			$ 1,000
Significant Other Observable Inputs (Level 2) | Convertible Notes | Face Value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	2,100,000	$ 2,100,000
Significant Other Observable Inputs (Level 2) | Convertible Notes | Estimated fair value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	4,300,000	3,000,000
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	194,525,000	163,674,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 194,525,000	$ 163,674,000

Summary of Significant Accou_14

Summary of Significant Accounting Policies - Leases (Details) $ in Thousands	Dec. 31, 2020USD ($)
Commitments and Contingencies
Obligations under finance leases	$ 0

Summary of Significant Accou_15

Summary of Significant Accounting Policies - Other Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other Accrued Liabilities
Accrued contract payments	$ 15,576	$ 5,188
Accrued clinical trial costs	11,401	6,418
Accrued professional services	1,200	1,274
Accrued employee benefits	39	314
Accrued public reporting charges	319	180
Other current accrued liabilities	785	558
Total	$ 29,320	$ 13,932

Summary of Significant Accou_16

Summary of Significant Accounting Policies - PPE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property and Equipment
Net gains (losses) related to impairment charges and the sale/disposal of furniture and equipment	$ 691	$ (1,689)	$ (115)
Asset Impairment Charges		2,500
Machinery and equipment
Property and Equipment
Estimated useful lives	5 years
Computer hardware and software
Property and Equipment
Estimated useful lives	3 years
Furniture and fixtures
Property and Equipment
Estimated useful lives	5 years
Net gains (losses) related to impairment charges and the sale/disposal of furniture and equipment	$ 700	$ (1,700)	$ (100)
Leasehold improvements | Maximum
Property and Equipment
Estimated useful lives	7 years

Summary of Significant Accou_17

Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)shares
Computation of Net Loss per Common Share
Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock at end of period	20,873	14,815	17,380
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock	1,301	1,020	3,001
Shares issuable upon conversion of convertible notes at end of period (in shares) | $	501	501	501
Common stock equivalents under if-converted method for convertible notes (in shares)	501	501	501

Summary of Significant Accou_18

Summary of Significant Accounting Policies - Stock-Based Compensation (Details)	1 Months Ended		12 Months Ended
	Sep. 30, 2018installmentshares	Jun. 30, 2018shares	Dec. 31, 2020USD ($)installmentitemplan$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Stock-Based Compensation
Number of employee share-based compensation plans | plan			3
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)				0.00%
Volatility (as a percent)				76.67%
Risk-free interest rate (as a percent)				2.20%
Expected life				6 years
Weighted-Average Remaining Life (in years)
Outstanding at the end of the period			7 years 7 months 28 days
Vested or unvested and expected to vest at the end of the period			7 years 7 months 9 days
Exercisable at the end of the period			5 years 9 months 25 days
Aggregate Intrinsic Value
Outstanding at the end of the period | $			$ 31,110,000
Vested or unvested and expected to vest at the end of the period | $			29,990,000
Exercisable at the end of the period | $			$ 9,222,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate number of common shares reserved for future issuance			25,000,000
Proceeds from Stock Options Exercised | $			$ 1,471,000	$ 2,873,000	$ 4,301,000
Total fair value of options vested | $			11,465,000	13,747,000	7,496,000
Total intrinsic value of options exercised | $			746,000	556,000	3,787,000
Directors' deferred share unit compensation | $			$ 343,000	$ 337,000	361,000
ESPP
Weighted-Average Grant Date Fair Value
Awarded (in dollars per share) | $ / shares			$ 1.20	$ 2.14
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate number of common shares reserved for future issuance		2,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent		85.00%
Shares, Issued			122,000	356,000
Awarded (in dollars per share) | $ / shares			$ 1.20	$ 2.14
Stock options and restricted stock awards
Stock-Based Compensation
Vesting period			2 years 7 months 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $			$ 14,000,000	$ 13,800,000	16,400,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $			17,800,000
Directors' deferred share unit compensation | $			$ 400,000	$ 300,000	$ 400,000
Stock options
Stock-Based Compensation
Share-based Compensation Arrangement by Share-based Payment Award					1,500,000
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)			0.00%		0.00%
Volatility (as a percent)			85.07%		71.02%
Risk-free interest rate (as a percent)			1.21%		2.73%
Expected life			6 years		6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares			$ 3.28	$ 2.81	$ 6.70
Number of Stock Options
Outstanding at the beginning of the period (in shares)			13,518,000
Granted (in shares)			7,421,000
Exercised (in shares)			(336,000)
Forfeited/Canceled (in shares)			(2,205,000)
Outstanding at the end of the period (in shares)			18,398,000	13,518,000
Vested or unvested and expected to vest at the end of the period (in shares)			17,830,000
Exercisable at the end of the period (in shares)			6,983,000	5,801,000	8,405,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 7.53
Granted (in dollars per share) | $ / shares			4.60
Exercised (in dollars per share) | $ / shares			2.94
Forfeited/Canceled (in dollars per share) | $ / shares			10.28
Outstanding at the end of the period (in dollars per share) | $ / shares			6.10	$ 7.53
Vested or unvested and expected to vest at the end of the period (in dollars per share) | $ / shares			6.15
Exercisable at the end of the period (in dollars per share) | $ / shares			$ 8.15	$ 10.16	$ 11.47
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Proceeds from Stock Options Exercised | $			$ 1,000,000
Stock options | Maximum
Stock-Based Compensation
Vesting period			4 years
Exercise period			10 years
Performance shares
Stock-Based Compensation
Number of equal installments | installment	2		4
Number of Stock Options
Granted (in shares)			2,600,000
Outstanding at the end of the period (in shares)			128,700
Number of Restricted Stock Shares
Awarded (in shares)	295,200		57,400
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $			$ 0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $			$ 9,400,000
Restricted stock
Stock-Based Compensation
Number of equal installments | item			3
Number of Restricted Stock Shares
Unvested at the beginning of the period (in shares)			1,297,000
Vested (in shares)			(749,000)
Forfeited (in shares)			(487,000)
Unvested at the end of the period (in shares)			61,000	1,297,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares			$ 2.97
Vested (in dollars per share) | $ / shares			2.60
Forfeited (in dollars per share) | $ / shares			3.62
Unvested at the end of the period (in dollars per share) | $ / shares			$ 2.47	$ 2.97
Restricted stock | Officers
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $			$ 0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $			$ 100,000
Immunogen Inc Restated Stock Option Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)			19,500,000
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)			7,500,000
2018 Plan | Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Number of group of awards for which expected term is calculated for and applied | $			1
Number of Stock Options
Exercised (in shares)			(336,000)

Summary of Significant Accou_19

Summary of Significant Accounting Policies - Segments (Details) - segment	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Information
Number of operating segments	1
Other customers
Segment Information
Percentages of revenue recognized	0.00%	0.00%	0.00%
CytomX
Segment Information
Percentages of revenue recognized		13.00%	8.00%
Roche
Segment Information
Percentages of revenue recognized	53.00%	64.00%	60.00%
Takeda
Segment Information
Percentages of revenue recognized	1.00%		23.00%
Jazz
Segment Information
Percentages of revenue recognized	46.00%	18.00%

Agreements - Roche (Details)

Agreements - Roche (Details) $ in Thousands	May 03, 2019USD ($)	Jan. 31, 2019USD ($)	May 31, 2000	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2000USD ($)item
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties				$ 68,529	$ 47,415	$ 32,154
Revenue recognized, previously deferred				61,872	14,817	14,139
Revenue from contract with customer				132,299	82,271	53,446
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer				63,742	34,788	15,280
Roche
Collaborative Agreements disclosures
Period in arrears to receive royalty reports and payments related to sales of Kadcyla			3 months
Percentage of royalty payments		100.00%
Roche | Upfront payment
Collaborative Agreements disclosures
Revenue from contract with customer							$ 2,000
Roche | License and milestone fees | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments							$ 44,000
Roche | License and milestone fees | Regulatory milestones
Collaborative Agreements disclosures
Revenue from contract with customer	$ 5,000
Roche | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties				68,500	$ 47,400	$ 32,200
Revenue from contract with customer				39,000
Roche | Kadcyla | Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments				5,000
Roche | Undisclosed Target
Collaborative Agreements disclosures
Number of undisclosed targets with exclusive licenses | item							4
Revenue from contract with customer							$ 1,000
Potential milestone payments							38,000
Roche | Undisclosed Target | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payments							28,000
Roche | Undisclosed Target | Milestone payments
Collaborative Agreements disclosures
Potential milestone payments							$ 10,000
Roche | Undisclosed Target | IND application filed
Collaborative Agreements disclosures
Potential milestone payments				$ 1,000
OMERS | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties		$ 65,200
Percentage of royalty payments		100.00%

Agreements - Amgen_Oxford BioTh

Agreements - Amgen/Oxford BioTherapeutics (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2000USD ($)item	Oct. 31, 2013USD ($)
Collaborative Agreements disclosures
Revenue from contract with customer	$ 132,299	$ 82,271	$ 53,446
Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Revenue from contract with customer	4,000
Number of single-target licenses | item				4
Number of licenses terminated | item				4
Amgen/Oxford BioTherapeutics | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment					$ 29,000
Amgen/Oxford BioTherapeutics | Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payment	3,000
Amgen/Oxford BioTherapeutics | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment					5,000
Upfront payment | Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Revenue from contract with customer				$ 1,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	$ 63,742	$ 34,788	$ 15,280
License and milestone fees | Amgen/Oxford BioTherapeutics
Collaborative Agreements disclosures
Potential milestone payment					$ 34,000

Agreements - Bayer (Details)

Agreements - Bayer (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2008
Collaborative Agreements disclosures
Total revenues	$ 132,299	$ 82,271	$ 53,446
Bayer | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment				$ 60,500
Bayer | Development milestones
Collaborative Agreements disclosures
Potential milestone payment	2,000
Bayer | Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment	6,000
Bayer | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment				110,000
Upfront payment | Bayer
Collaborative Agreements disclosures
Total revenues				4,000
License and milestone fees
Collaborative Agreements disclosures
Total revenues	63,742	$ 34,788	$ 15,280
License and milestone fees | Bayer
Collaborative Agreements disclosures
Total revenues	$ 13,000
Potential milestone payment				$ 170,500

Agreements - Novartis (Details)

Agreements - Novartis (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Sep. 30, 2020USD ($)	May 31, 2018item	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2010USD ($)item
Collaborative Agreements disclosures
Revenue from contract with customer			$ 132,299	$ 82,271	$ 53,446
Contract with Customer, Liability			$ 110,109	127,432	80,802
Novartis
Collaborative Agreements disclosures
Number of single-target licenses | item			5					6
Number of licenses terminated | item		6
Novartis | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment			$ 99,500
Novartis | Development milestones
Collaborative Agreements disclosures
Revenue from contract with customer						$ 5,000	$ 5,000
Novartis | Phase 1 clinical trial
Collaborative Agreements disclosures
Revenue from contract with customer	$ 5,000			4,700
Potential milestone payment			5,000	5,000
Contract with Customer, Liability			300
Novartis | Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payment			7,500
Novartis | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment			100,000
Upfront payment | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer								$ 45,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer			63,742	34,788	15,280
License and milestone fees | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer					1,000
Potential milestone payment								199,500
Exercise fee | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer								1,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer			$ 28	$ 68	$ 1,377
Extension and amendment fee | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer								$ 8,500

Agreements - CytomX (Details)

Agreements - CytomX (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2019USD ($)item	Jun. 30, 2017USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)item	Jun. 30, 2016USD ($)	Jan. 31, 2014USD ($)
Collaborative Agreements disclosures
Revenue from contract with customer			$ 132,299	$ 82,271	$ 53,446
CytomX
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross								$ 160,000
Number of licenses terminated | item	1					1
CytomX | Development and regulatory milestones
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross							$ 60,000
CytomX | Development milestones
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross								60,000
CytomX | Phase 2 clinical trial
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross			6,000
CytomX | Phase 1 clinical trial
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross			1,000
CytomX | Milestone payments
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross							100,000	$ 100,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer			28	68	1,377
Research and development support | CytomX
Collaborative Agreements disclosures
Cost of Goods Sold					200
Clinical materials revenue
Collaborative Agreements disclosures
Revenue from contract with customer					4,635
Clinical materials revenue | CytomX
Collaborative Agreements disclosures
Cost of Goods Sold			0		3,500
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer			63,742	34,788	$ 15,280
License and milestone fees | CytomX
Collaborative Agreements disclosures
Revenue from contract with customer	$ 7,300					$ 12,700
License and milestone fees | CytomX | Maximum
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross			355,000
License and milestone fees | CytomX | Development and regulatory milestones
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross			205,000
License and milestone fees | CytomX | Phase 2 clinical trial
Collaborative Agreements disclosures
Cost of Goods Sold				$ 3,000
License and milestone fees | CytomX | Phase 1 clinical trial
Collaborative Agreements disclosures
Revenue from contract with customer		$ 1,000
License and milestone fees | CytomX | Milestone payments
Collaborative Agreements disclosures
Contract with Customer, Asset, Gross			$ 150,000				$ 160,000

Agreements - Fusion (Details)

Agreements - Fusion (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Collaborative Agreements disclosures
Revenue from contract with customer	$ 132,299	$ 82,271	$ 53,446
Fusion Pharmaceuticals | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment				$ 15,000
Fusion Pharmaceuticals | Phase 2 clinical trial
Collaborative Agreements disclosures
Potential milestone payment		1,500
Fusion Pharmaceuticals | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment				35,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	63,742	34,788	15,280
License and milestone fees | Fusion Pharmaceuticals
Collaborative Agreements disclosures
Potential milestone payment				$ 50,000
License and milestone fees | Fusion Pharmaceuticals | Phase 1 clinical trial
Collaborative Agreements disclosures
Revenue from contract with customer			500
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer	$ 28	$ 68	1,377
Clinical materials revenue
Collaborative Agreements disclosures
Revenue from contract with customer			$ 4,635

Agreements - Debiopharm (Detail

Agreements - Debiopharm (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	May 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Agreements disclosures
Revenue from contract with customer				$ 132,299	$ 82,271	$ 53,446
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer				63,742	34,788	15,280
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer				28	$ 68	1,377
Debiopharm | IMGN529 program
Collaborative Agreements disclosures
Revenue from contract with customer	$ 500	$ 4,500
Debiopharm | Phase 3 Clinical Trial
Collaborative Agreements disclosures
Potential milestone payment				$ 25,000
Debiopharm | Transfer of ImmunoGen technologies
Collaborative Agreements disclosures
Potential milestone payment			$ 5,000
Debiopharm | Upfront payment | IMGN529 program
Collaborative Agreements disclosures
Revenue from contract with customer			$ 25,000
Debiopharm | License and milestone fees | IMGN529 program
Collaborative Agreements disclosures
Revenue from contract with customer						$ 500	$ 29,500

Agreements - Viridian (Details)

Agreements - Viridian (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Agreements disclosures
Revenue from contract with customer		$ 132,299	$ 82,271	$ 53,446
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		$ 63,742	$ 34,788	$ 15,280
Viridian | Development and regulatory milestones
Collaborative Agreements disclosures
Revenue from contract with customer	$ 48,000
Viridian | Milestone payments
Collaborative Agreements disclosures
Revenue from contract with customer	95,000
Viridian | License and milestone fees
Collaborative Agreements disclosures
Potential milestone payment	$ 143,000

Agreements - Huadong (Details)

Agreements - Huadong (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Agreements disclosures
Revenue from contract with customer		$ 132,299	$ 82,271	$ 53,446
Revenue recognized, previously deferred		61,872	14,817	14,139
Huadong
Collaborative Agreements disclosures
Revenue recognized, previously deferred		0
Huadong | Development and regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment	$ 80,000
Huadong | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment	185,000
Upfront payment | Huadong
Collaborative Agreements disclosures
Revenue from contract with customer	40,000
Estimated payments to received	40,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		$ 63,742	$ 34,788	$ 15,280
License and milestone fees | Huadong | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment	$ 265,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 | Huadong
Collaborative Agreements disclosures
Remaining performance obligation, expected timing of satisfaction		2 years

Agreements - Jazz Pharmaceutica

Agreements - Jazz Pharmaceuticals (Details) $ in Thousands	Aug. 31, 2017USD ($)Option	Aug. 31, 2017USD ($)item	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Collaborative Agreements disclosures
Revenue from contract with customer			$ 132,299	$ 82,271	$ 53,446
Deferred revenue			110,109	127,432	80,802
Revenue recognized, previously deferred			61,872	14,817	14,139
Jazz
Collaborative Agreements disclosures
Number of early stage ADC programs | item		2
Potential milestone payment	$ 100,000	$ 100,000
Term of agreement		7 years
Offset to research and development expense			6,700	12,500	10,000
Revenue recognized, previously deferred				14,500
Upfront payment | Jazz
Collaborative Agreements disclosures
Revenue from contract with customer		$ 75,000
Upfront Arrangement Consideration, Number of License Options allocated To | Option	3
Deferred revenue			60,500
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer			63,742	34,788	15,280
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer			$ 28	$ 68	$ 1,377

Agreements - Takeda (Details)

Agreements - Takeda (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	May 31, 2018	Mar. 31, 2018	Mar. 31, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2016
Collaborative Agreements disclosures
Revenue from contract with customer				$ 132,299	$ 82,271	$ 53,446
ASU 2014-09 | Adjustments due to new guidance
Collaborative Agreements disclosures
Revenue from contract with customer						5,000
Takeda | Phase 1 clinical trial | ASU 2014-09 | Adjustments due to new guidance
Collaborative Agreements disclosures
Revenue from contract with customer	$ 5,000
Takeda | Right-to-test agreement
Collaborative Agreements disclosures
Term of agreement			3 years
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer				63,742	34,788	15,280
License and milestone fees | Takeda
Collaborative Agreements disclosures
Revenue from contract with customer		$ 10,900		900			$ 8,600
Upfront payment | Takeda
Collaborative Agreements disclosures
Revenue from contract with customer			$ 20,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer				$ 28	$ 68	1,377
Clinical materials revenue
Collaborative Agreements disclosures
Revenue from contract with customer						$ 4,635

Agreements - Lilly (Details)

Agreements - Lilly (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2018item	Dec. 31, 2020USD ($)item	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Collaborative Agreements disclosures
Revenue from contract with customer		$ 132,299	$ 82,271	$ 53,446
Remaining arrangement consideration to be recognized as license revenue		80,860	127,123
Expense related to excess inventory		114,592	114,522	174,456
Lilly
Collaborative Agreements disclosures
Revenue from contract with customer		$ 20,000
Number of development and commercialization licenses | item		3
Number of licenses terminated | item	3
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer		$ 63,742	34,788	15,280
License and milestone fees | Lilly
Collaborative Agreements disclosures
Revenue from contract with customer				700
Exercise fee | Lilly
Collaborative Agreements disclosures
Revenue from contract with customer		4,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer		$ 28	$ 68	1,377
Clinical materials revenue
Collaborative Agreements disclosures
Revenue from contract with customer				$ 4,635

Property and Equipment (Details

Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property and Equipment
Property and equipment, gross	$ 39,459		$ 33,523	$ 39,459
Less accumulated depreciation	(32,466)		(27,763)	(32,466)
Property and equipment, net	6,993		5,760	6,993
Depreciation expense			2,101	4,028	$ 7,411
Impairment charge to write down excess equipment to fair value		$ 2,500
Equipment liquidate amount	2,000
Agreement to liquidate equipment, cost basis	14,200		6,700
Accumulated depreciation and impairment charges for liquidated equipment	12,900
Payment from equipment held for sale			1,200
Leasehold improvements
Property and Equipment
Property and equipment, gross	20,776		21,890	20,776
Machinery and equipment
Property and Equipment
Property and equipment, gross	9,384		2,861	9,384
Computer hardware and software
Property and Equipment
Property and equipment, gross	5,692		5,636	5,692
Furniture and fixtures
Property and Equipment
Property and equipment, gross	3,607		3,039	3,607
Assets under construction
Property and Equipment
Property and equipment, gross			97
Equipment under capital leases
Property and Equipment
Property and equipment, gross	2,100		2,100	2,100
Less accumulated depreciation	$ (1,000)		$ (1,300)	$ (1,000)

Convertible 4.5% Senior Notes (

Convertible 4.5% Senior Notes (Details)	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2017	Dec. 31, 2016USD ($)$ / shares
Convertible debt
Interest rate (as a percent)	4.50%	4.50%
Interest expense	$ 95,000	$ 95,000	$ 95,000
Convertible Notes
Convertible debt
Principal amount of debt						$ 100,000,000
Convertible 4.5% Senior Notes outstanding				$ 2,100,000
Interest rate (as a percent)	4.50%	4.50%			4.50%
Interest expense	$ 100,000	$ 100,000	$ 100,000
Principal amount of debt for conversion calculations	$ 1,000					$ 1,000
Ratio issued upon conversion			238.7775
Initial conversion price (in dollars per share) | $ / shares						$ 4.19

Liability Related to Sale of _3

Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended				72 Months Ended
	Jan. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2015	Dec. 31, 2020
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 68,529	$ 47,415	$ 32,154
Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments if applicable threshold is met					85.00%
IRH | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments					100.00%
Percentage of royalty payments if applicable threshold is met					15.00%
Proceeds from sale of future royalties - net		200,000			$ 200,000	$ 194,135
Transaction costs for royalty agreements					5,900
Change in liability related to sale of future royalties
Liability related to sale of future royalties, net - beginning balance		123,541
Royalty payments received and paid		(61,195)				(206,367)
Non-cash interest expense recognized		23,093				97,671
Liability related to sale of future royalties, net - ending balance		$ 85,439	$ 123,541			$ 85,439
Effective annual interest rate		10.50%
Current effective interest rate		22.2
IRH | Kadcyla | Maximum
Liability Related to Sale of Future Royalties
Royalties threshold					260,000
IRH | Kadcyla | Minimum
Liability Related to Sale of Future Royalties
Royalties threshold					$ 235,000
OMERS | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments	100.00%
Non-cash royalty revenue related to the sale of future royalties	$ 65,200
Contingent broker fees	1,500
Net proceeds from sale of residual rights to receive royalty payments	$ 65,200	$ 0

Income Taxes - Benefit for inco

Income Taxes - Benefit for income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the expected statutory tax benefit to the actual income taxes
U.S. federal corporate tax rate (as a percent)	21.00%
Loss before income tax expense	$ (44,372)	$ (104,133)	$ (168,843)
Expected tax benefit at 21%, 34%, 34% and 34%, respectively	(9,318)	(21,868)	(35,457)
Permanent differences	157	320	(103)
Incentive stock options	201	569	1,144
State tax benefit net of federal benefit	(2,250)	(6,726)	(10,622)
Change in valuation allowance, net	15,175	27,812	53,706
Federal research credit	(228)	(1,652)	(2,466)
Federal orphan drug credit	(6,218)	(4,426)	(6,934)
Expired loss and credit carryforwards	419	500
Lease incentive			109
Stock option expirations	2,062	5,471	623
Benefit for income taxes	$ 0	$ 0	$ 0

Income Taxes - Carryforward (De

Income Taxes - Carryforward (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Federal
Carryforward
Operating loss carryforward	$ 471.6
NOL carryforwards, indefinitely	373.7
Credit carryforwards	70.4
State
Carryforward
Operating loss carryforward	677.1
Credit carryforwards	$ 13

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets/Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 220,286	$ 191,744
Research and development tax credit carryforwards	80,694	75,084
Property and other intangible assets	871	809
Deferred revenue	30,082	36,008
Stock-based compensation	9,940	9,630
Operating Lease Liability	6,033	6,767
Other liabilities	1,514	2,255
Royalty sale	17,455	30,030
Total deferred tax assets	366,875	352,327
Deferred tax liabilities:
Stock-based compensation	(58)	(110)
Operating lease right of use asset	(3,844)	(4,258)
Royalty sale transaction costs	(247)	(408)
Total deferred tax liabilities	4,149	4,776
Valuation allowance	$ (362,726)	$ (347,551)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2015
Tax Cuts and Jobs Act (TCJA)
Increase in valuation allowance	$ 15,200,000
U.S. federal corporate tax rate (as a percent)	21.00%
Income Tax Uncertainties
Ownership change occurred, based off study			0
Uncertain tax positions reported	$ 0	$ 0

Capital Stock (Details)

Capital Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based compensation disclosure
Aggregate number of common shares reserved for future issuance	25,000,000
Proceeds from issuance of common stock under stock plans	$ 1,471	$ 2,873	$ 4,301
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)	7,421,000
Options exercised (in shares)	336,000
Exercise price (in dollars per share)	$ 2.31
Exercise price (in dollars per share)	$ 5.25
Proceeds from issuance of common stock under stock plans	$ 1,000
Exercisable at the end of the period (in shares)	6,983,000	5,801,000	8,405,000
Weighted-Average Exercise Price (in dollars per share)	$ 8.15	$ 10.16	$ 11.47
2018 Plan | Stock options
Stock-based compensation disclosure
Options exercised (in shares)	336,000
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)	300,000	108,000	128,000
Compensation Policy for Non-Employee Directors | Deferred share units
Stock-based compensation disclosure
Compensation expense	$ 300	$ 300	$ 400
Common stock issued to retiring directors (in shares)	127,000	63,000	46,000
Common stock issued when a director ceases to be a member (in shares)	1
Stock units previously granted (in shares)	15,000	18,000	10,500
Compensation Policy for Non-Employee Directors | Deferred share units | Retiring director
Stock-based compensation disclosure
Common stock issued to retiring directors (in shares)			172,509

Restructuring Charge (Details)

Restructuring Charge (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Feb. 28, 2018employee	Jun. 30, 2019USD ($)	Mar. 31, 2018USD ($)	Jun. 30, 2019USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2020USD ($)ft²	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Restructuring
Severance costs, charges incurred						$ 2,300
Restructuring charge
Restructuring charge							$ 1,487	$ 21,433	$ 3,693
Workforce reduction
Restructuring charge
Balance at beginning of the period							4,087
Additional charges/adjustments during the period				$ 15,300			(116)
Payments during the period							(3,187)
Balance at end of the period							$ 784	4,087
Workforce reduction | 2019 Corporate Restructuring
Restructuring
Charge for severance related to a pre-existing plan		$ 16,000
Workforce reduction | 2018 Manufacturing Restructuring
Restructuring
Number of employees eliminated due to new operating model | employee	22
Workforce reduction | 2018 Manufacturing Restructuring | Stock options
Restructuring
Severance costs, charges incurred			$ 200
Workforce reduction | 2016 Corporate Restructuring
Restructuring charge
Unoccupied office space for sub-lease | ft²							10,281
Lease | 2016 Corporate Restructuring
Restructuring charge
Leasehold impairment charge								600
One-time charge | 2018 Manufacturing Restructuring
Restructuring
Severance costs, charges incurred			$ 1,200
Incremental retention benefits | 2019 Corporate Restructuring
Restructuring
Severance costs, charges incurred		4,000					$ 1,600	$ 2,400
Winter Street 930 Waltham MA
Restructuring charge
Unoccupied office space for sub-lease | ft²							10,281
Leasehold impairment charge					$ 600
Winter Street 930 Waltham MA | Lease | 2019 Corporate Restructuring
Restructuring charge
Impairment charge to write down excess equipment to fair value		$ 2,500

Leases - Operating Leases (Deta

Leases - Operating Leases (Details) $ in Thousands	Jan. 01, 2019USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2020USD ($)ft²	Dec. 31, 2020USD ($)ft²	Dec. 31, 2020USD ($)ft²lease	Dec. 31, 2020USD ($)ft²	Dec. 31, 2020USD ($)ft²item	Dec. 31, 2020USD ($)ft²agreement	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Lessee, Lease, Description [Line Items]
Number of real estate leases					2		2
Right-of-use assets			$ 14,072	$ 14,072	$ 14,072	$ 14,072	$ 14,072	$ 14,072	$ 15,587
Lease liabilities			$ 21,797	$ 21,797	$ 21,797	$ 21,797	$ 21,797	$ 21,797
Unamortized lease incentive and straight-line lease liability balances	$ 9,700
Weighted-average discount rate			11.00%	11.00%	11.00%	11.00%	11.00%	11.00%
100 basis point change effect on ROU asset				$ 1,000
Lease expense for operating lease payments				4,000					4,300	$ 5,800
Operating lease right-of-use asset impairment									694
Cash paid against operating lease liabilities				$ 5,500					$ 5,300
Weighted average remaining term of the operating leases			5 years	5 years	5 years	5 years	5 years	5 years
Sublease income				$ 2,800
Lease, Practical Expedients, Package [true false]			true
Lessee, Operating Lease, Existence of Option to Extend [true false]			true
CRP/King 830 Winter L.L.C.
Lessee, Lease, Description [Line Items]
Area of space leased | ft²			120,000	120,000	120,000	120,000	120,000	120,000
Number of additional terms for which lease agreement can be extended | item							2
Operating lease term extension period			5 years	5 years	5 years	5 years	5 years	5 years
Number of executed sub-lease spaces | lease					4
Area of executed sublease space | ft²						65,000
PDM 930 Unit, LLC
Lessee, Lease, Description [Line Items]
Area of executed sublease space | ft²						10,281
Winter Street 930 Waltham MA
Lessee, Lease, Description [Line Items]
Number of executed sub-lease spaces | agreement								4
Area of executed sublease space | ft²						65,000
Operating lease right-of-use asset impairment				$ 600
Leasehold impairment charge		$ 600
Minimum rental payments over the remaining term of the sublease			$ 15,900	$ 15,900	$ 15,900	$ 15,900	$ 15,900	$ 15,900
ASU 2016-2 | Restatement Adjustment
Lessee, Lease, Description [Line Items]
Right-of-use assets	17,600
Lease liabilities	$ 27,300

Leases - Finance Lease (Details

Leases - Finance Lease (Details) $ in Thousands	Dec. 31, 2020USD ($)
Leases
Finance Lease, Liability	$ 0

Leases - Maturities of Operatin

Leases - Maturities of Operating Lease Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Leases
2021	$ 5,323
2022	5,389
2023	5,510
2024	5,470
2025	5,490
Thereafter	1,376
Total lease payments	28,558
Less imputed interest	(6,761)
Total lease liabilities	21,797
Variable operating costs and real estate taxes	$ 3,100

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Dec. 31, 2020USD ($)
In-process and future manufacturing of antibody and cytotoxic agents
Collaborations and Manufacturing Commitments
Noncancelable obligations under several agreements	$ 6.5
Minimum
Collaborations and Manufacturing Commitments
Manufacturing commitment	$ 36

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Benefit Plans
Maximum employees' contribution (as a percent)	100.00%
Matching contribution of first 6% of eligible employees' contributions (as a percent)	50.00%
Percentage of eligible employees' contributions matched by the company	6.00%
Company's contribution	$ 0.4	$ 0.8	$ 1