Epizyme (EPZM) 10-K 23 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 18, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	EPZM
Security Exchange Name	NASDAQ
Entity File Number	001-35945
Title of 12(b) Security	Common stock, $0.0001 par value
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	0001571498
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Document Annual Report	true
Document Transition Report	false
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	true
Entity Common Stock Shares Outstanding		101,785,162
Entity Public Float			$ 794.6
Entity Tax Identification Number	26-1349956
Entity Address, Address Line One	400 Technology Square
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Incorporation, State or Country Code	DE
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	229-5872
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement that the registrant intends to file with the Securities and Exchange Commission pursuant to Regulation 14A in connection with the registrant’s 2021 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K to the extent stated herein.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 168,215	$ 139,482
Marketable securities	205,391	241,605
Accounts receivable, net	3,105	2,567
Inventory	10,461
Prepaid expenses and other current assets	17,921	15,523
Total current assets	405,093	399,177
Property and equipment, net	2,152	2,219
Operating lease assets	17,305	21,206
Intangible assets, net	47,002
Restricted cash and other assets	2,021	1,987
Total assets	473,573	424,589
Current liabilities:
Accounts payable	10,163	8,782
Accrued expenses	28,572	22,565
Current portion of operating lease obligation	4,665	3,039
Total current liabilities	43,400	34,386
Operating lease obligation, net of current portion	15,409	19,120
Deferred revenue, net of current portion		3,806
Related party long-term debt, net of debt discount	215,670	23,309
Other long-term liabilities	21	38
Related party liability related to sale of future royalties	14,176	12,793
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; 338 shares and 350 shares issued and outstanding, respectively (equivalent to 3,378 shares and 3,500 shares of common stock, respectively, upon conversion at a 10:1 ratio)	36,127	37,432
Common stock, $0.0001 par value; 150,000 shares and 125,000 shares, respectively, authorized; 101,627 shares and 97,783 shares issued and outstanding, respectively	10	10
Additional paid-in capital	1,137,470	1,050,695
Accumulated other comprehensive loss	3	19
Accumulated deficit	(988,713)	(757,019)
Total stockholders’ equity	184,897	331,137
Total liabilities and stockholders’ equity	$ 473,573	$ 424,589

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 30, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	338,000	350,000
Preferred stock, shares outstanding	338,000	350,000
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	125,000,000
Common stock, shares issued	101,627,000	97,783,000
Common stock, shares outstanding	101,627,000	97,783,000
Common stock conversion ratio	10.00%
Common stock upon conversion	3,378,000	3,500,000

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
Revenue:
Total revenue	$ 15,762	$ 23,800	$ 21,700
Operating expenses:
Cost of product revenue	5,067
Research and development	110,933	132,639	105,833
Selling, general and administrative	125,178	68,303	43,972
Total operating expenses	241,178	200,942	149,805
Operating loss	(225,416)	(177,142)	(128,105)
Other (expense) income, net:
Interest (expense) income, net	(4,682)	7,110	4,557
Other expense, net	(99)	(13)	(25)
Related party non-cash interest expense related to sale of future royalties	(1,383)	(192)
Other (expense) income, net	(6,164)	6,905	4,532
Loss before income taxes	(231,580)	(170,237)	(123,573)
Income tax provision	(114)	(58)	(57)
Net loss	(231,694)	(170,295)	(123,630)
Other comprehensive (loss) income:
Unrealized gain (loss) on available for sale securities	(16)	73	(5)
Comprehensive loss	(231,710)	(170,222)	(123,635)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(231,694)	(170,295)	(123,630)
Accretion of convertible preferred stock		(2,940)
Net loss attributable to common stockholders	$ (231,694)	$ (173,235)	$ (123,630)
Net loss per share attributable to common stockholders:
Basic	$ (2.29)	$ (1.93)	$ (1.72)
Diluted	$ (2.29)	$ (1.93)	$ (1.72)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders:
Basic	100,960	89,891	71,864
Diluted	100,960	89,891	71,864
Product [Member]
Revenue:
Total revenue	$ 11,469
Collaboration Revenue [Member]
Revenue:
Total revenue	$ 4,293	$ 23,800	$ 21,700

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (231,694,000)	$ (170,295,000)	$ (123,630,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,984,000	840,000	1,052,000
Stock-based compensation	27,609,000	18,016,000	12,004,000
Amortization of discount on investments	(93,000)	(3,175,000)	(1,556,000)
Amortization of debt discount	406,000	37,000
Loss on disposal of property and equipment	19,000
Non-cash interest expense associated with the sale of future royalties	1,383,000	192,000
Deferred income taxes	92,000	92,000	184,000
Changes in operating assets and liabilities:
Accounts receivable	(538,000)	17,500,000	(19,686,000)
Inventory	(10,461,000)
Prepaid expenses and other current assets	(2,398,000)	(3,359,000)	(3,181,000)
Accounts payable	1,480,000	3,389,000	(2,404,000)
Accrued expenses	6,006,000	2,897,000	2,066,000
Deferred revenue	(3,806,000)	(13,300,000)	13,300,000
Operating lease assets	3,901,000	(9,921,000)
Operating lease liabilities	(2,085,000)	10,043,000
Other assets and liabilities	(141,000)	(124,000)	251,000
Net cash used in operating activities	(206,336,000)	(147,168,000)	(121,600,000)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available-for-sale securities	(276,404,000)	(504,981,000)	(298,670,000)
Maturities of available-for-sale securities	312,694,000	420,255,000	196,363,000
Purchase of intangible asset	(50,000,000)
Purchases of property and equipment	(880,000)	(594,000)	(299,000)
Net cash used in investing activities	(14,590,000)	(85,320,000)	(102,606,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of commissions		122,991,000	81,938,000
Proceeds from issuance of preferred stock, net of commissions		37,432,000
Payment of offering costs	(79,000)	(284,000)	(260,000)
Proceeds from the issuance of debt	195,000,000	25,000,000
Payment of debt issuance costs	(3,123,000)	(1,650,000)
Proceeds from the issuance of common stock, warrants, and sale of future royalties to RPI, net of offering costs		99,774,000
Proceeds from the issuance of common stock in connection with the exercise of the Put Option, net of financing costs	49,915,000
Payment under capital lease obligation		(16,000)	(129,000)
Proceeds from stock options exercised	6,692,000	2,358,000	1,885,000
Issuance of shares under employee stock purchase plan	1,254,000	741,000	779,000
Net cash provided by financing activities	249,659,000	286,346,000	84,213,000
Net increase (decrease) in cash, cash equivalents and restricted cash	28,733,000	53,858,000	(139,993,000)
Cash, cash equivalents, and restricted cash, beginning of period	140,991,000	87,133,000	227,126,000
Cash, cash equivalents, and restricted cash, end of period	169,724,000	140,991,000	87,133,000
SUPPLEMENTAL CASH FLOW INFORMATION:
Unpaid offering costs		78,000	75,000
Unpaid debt issuance costs		78,000
Interest paid	7,461,000
Property and equipment included in accounts payable or accruals	60,000	454,000	194,000
Cash paid for income taxes	$ 64,000	$ 45,000	$ 48,000

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Put Option	Royalty Pharma [Member]	Series A Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Put Option	Common Stock [Member]Royalty Pharma [Member]	Common Stock [Member]Series A Convertible Preferred Stock [Member]	Preferred Stock [Member]	Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member]Put Option	Additional Paid-In Capital [Member]Royalty Pharma [Member]	Additional Paid-In Capital [Member]Series A Convertible Preferred Stock [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance, Value at Dec. 31, 2017	$ 235,371				$ 7						$ 723,510				$ (488,097)	$ (49)
Beginning Balance, Shares at Dec. 31, 2017					69,301,691
Cumulative catch up related to the adoption of ASU | ASU 2016-09 [Member]	25,003														25,003
Issuance of common stock (net of commissions and offering costs), Value	81,602				$ 1						81,601
Issuance of common stock (net of commissions and offering costs), Shares					9,583,334
Exercise of stock options and vesting of restricted stock units, Value	1,885										1,885
Exercise of stock options and vesting of restricted stock units, Shares					215,156
Stock-based compensation	11,839										11,839
Stock in lieu of board fees	165				$ 12,213						165
Issuance of shares under employee stock purchase plan, Value	779										779
Issuance of shares under employee stock purchase plan, Shares					62,986
Unrealized gain (loss) on available for sale securities	(5)															(5)
Net loss	(123,630)														(123,630)
Ending Balance, Value at Dec. 31, 2018	233,009				$ 8						819,779				(586,724)	(54)
Ending Balance, Shares at Dec. 31, 2018					79,175,380
Issuance of series A convertible preferred stock, net of commissions and beneficial conversion charge, Value				$ 37,432						$ 34,492				$ 2,940
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares										350,000
Accretion of series A convertible preferred stock	2,940									$ 2,940				(2,940)
Issuance of common stock (net of commissions and offering costs), Value	122,708		$ 78,705		$ 1		$ 1				122,707		$ 78,704
Issuance of common stock (net of commissions and offering costs), Shares					11,500,000		6,666,667
Issuance of warrant			$ 8,390										$ 8,390
Exercise of stock options and vesting of restricted stock units, Value	2,358										2,358
Exercise of stock options and vesting of restricted stock units, Shares					356,538
Stock-based compensation	17,875										17,875
Stock in lieu of board fees	141										141
Stock in lieu of board fees, Shares					12,156
Issuance of shares under employee stock purchase plan, Value	741										741
Issuance of shares under employee stock purchase plan, Shares					72,735
Unrealized gain (loss) on available for sale securities	73															73
Net loss	(170,295)														(170,295)
Ending Balance, Value at Dec. 31, 2019	331,137				$ 10				$ 37,432		1,050,695				(757,019)	19
Ending Balance, Shares at Dec. 31, 2019					97,783,476				350,000
Issuance of series A convertible preferred stock, net of commissions and beneficial conversion charge, Value										$ (1,305)				$ 1,305
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares								122,000		(12,200)
Issuance of common stock (net of commissions and offering costs), Value		$ 49,915										$ 49,915
Issuance of common stock (net of commissions and offering costs), Shares						2,500,000
Exercise of stock options and vesting of restricted stock units, Value	6,692										6,692
Exercise of stock options and vesting of restricted stock units, Shares					1,097,280
Stock-based compensation	27,471										27,471
Stock in lieu of board fees	138										138
Stock in lieu of board fees, Shares					8,683
Issuance of shares under employee stock purchase plan, Value	1,254										1,254
Issuance of shares under employee stock purchase plan, Shares					115,631
Unrealized gain (loss) on available for sale securities	(16)															(16)
Net loss	(231,694)														(231,694)
Ending Balance, Value at Dec. 31, 2020	$ 184,897				$ 10				$ 36,127		$ 1,137,470				$ (988,713)	$ 3
Ending Balance, Shares at Dec. 31, 2020					101,627,070				337,800

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Issuance of common stock, commissions and offering costs		$ 284	$ 388
Put Option
Financing Cost	$ 85
Royalty Pharma [Member]
Issuance of common stock, commissions and offering costs		$ 304

The Company

The Company	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company Epizyme, Inc. (collectively referred to with its wholly owned, controlled subsidiary, Epizyme Securities Corporation, as “Epizyme” or the “Company”) is a commercial-stage biopharmaceutical company that is committed to rewriting treatment for people with cancer and other serious diseases through the discovery, development, and commercialization of novel epigenetic medicines. By focusing on the genetic drivers of disease, the Company’s science seeks to match targeted medicines with the patients who need them. Through December 31, 2020, in addition to revenues from product sales, the Company has raised, an aggregate of $1,527.4 million to fund its operations. This includes $243.8 million of non-equity funding through its collaboration agreements, $368.1 million of funding, consisting of $150.0 million in equity funding received through agreements with RPI Finance Trust, or RPI, and $218.1 million in debt financing received through a loan agreement with BioPharma Credit Investments V (Master) LP and BPCR Limited Partnership (as transferee of BioPharma Credit Investments V (Master) LP’s interest as a lender), or the Lenders, $839.5 million from the sale of common stock and series A convertible preferred stock in the Company’s public offerings and $76.0 million was from the sale of redeemable convertible preferred stock in private financings prior to the Company’s initial public offering in May 2013. As of December 31, 2020, the Company had $373.6 million in cash, cash equivalents and marketable securities. In 2020, the Company’s EZH2 inhibitor, tazemetostat, was approved in the United States as TAZVERIK for the treatment of epithelioid sarcoma, or ES, and follicular lymphoma, or FL. Commercial sales of TAZVERIK for the treatment of ES commenced in the first quarter of 2020 and commercial sales of TAZVERIK for the treatment of two FL indications commenced near the end of the second quarter of 2020. The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $988.7 million through December 31, 2020 and will require substantial additional capital to fund its research, development, and commercialization efforts. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risks of failure of commercialization, clinical trials and preclinical studies, the need to obtain additional financing to fund the future development and commercialization of tazemetostat and the rest of its pipeline, the need to obtain marketing approval for its product candidates, the need to successfully commercialize and gain market acceptance of its product candidates, the impact of the COVID-19 pandemic on the Company’s business, results of operations, and financial condition, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations and ability to transition from clinical-stage manufacturing to commercial-stage production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The consolidated financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC, and in accordance with U.S. generally accepted accounting principles, or GAAP. Any reference in these notes to applicable guidance is meant to refer to the authoritative accounting principles generally accepted in the United States as found in the ASC and Accounting Standards Update, or ASU, of the Financial Accounting Standards Board, or FASB. The consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. Use of Estimates The preparation of these consolidated financial statements in accordance with accounting principles generally accepted in the United States requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the consolidated financial statements, and the reported amounts of revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions. Subsequent Events The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company evaluated all events and transactions through the date these financial statements were filed with the Securities and Exchange Commission. Cash and cash equivalents The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents are comprised of demand deposit accounts, funds in money market accounts, commercial paper and corporate notes. Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. The Company considers all available-for-sale securities, including those with maturity dates beyond 12 months, as available to support current operational liquidity needs and therefore classifies all securities with maturity dates beyond 90 days at the date of purchase as current assets. Available-for-sale securities are maintained by the Company’s investment managers and may consist of commercial paper, high-grade corporate notes, U.S. Treasury securities, and U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive loss as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2020 was $67.7 million, which consisted of 6 commercial paper securities, 7 corporate notes securities and 1 U.S. Treasury security. There were no marketable securities held by the Company for greater than twelve months as of December 31, 2020. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2019 was $116.7 million, which consisted of 3 commercial paper securities and 19 corporate notes securities. The aggregate fair value of securities held by the Company in an unrealized loss position for greater than twelve months as of December 31, 2019 was $4.0 million, which consisted of 1 U.S. government agency security. The Company does not intend to sell and it is unlikely that the Company will be required to sell the above investments before recovery of their amortized cost bases, which may be maturity. The Company determined there was no material change in the credit risk of the above investments, and as a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of December 31, 2020 and 2019. The Company reviews its portfolio of available-for-sale debt securities, using both quantitative and qualitative factors, to determine if declines in fair value below cost have resulted from a credit-related loss or other factors. If the decline in fair value is due to credit-related factors, a loss is recognized in net income, whereas if the decline in fair value is not due to credit-related factors, the loss is recorded in other comprehensive income (loss). The following table summarizes the available for sale securities held at December 31, 2020 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 158,907 $ 14 $ (8 ) $ 158,913 Corporate notes 33,437 3 (7 ) 33,433 U.S. government agency securities and U.S. Treasuries 13,044 1 — 13,045 Total $ 205,388 $ 18 $ (15 ) $ 205,391 The following table summarizes the available for sale securities held at December 31, 2019 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 96,952 $ 27 $ (16 ) $ 96,963 Corporate notes 140,634 49 (41 ) 140,642 U.S. government agency securities and U.S. Treasuries 4,000 — — 4,000 Total $ 241,586 $ 76 $ (57 ) $ 241,605 At December 31, 2020 and 2019, the Company had no securities and 1 security in an unrealized loss position for greater than twelve months, respectively, which the Company concluded was not impaired. Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. All marketable securities held at December 31, 2020 have maturities of less than one year. The majority of marketable securities held at December 31, 2019 have maturities of less than one year, with the exception of one U.S. government agency security. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2020, the balance in the Company’s accumulated other comprehensive loss was composed mainly of activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the year ended December 31, 2020 and December 31, 2019, respectively, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. Restricted Cash A reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows, is as follows: December 31, 2020 2019 2018 (In thousands) Cash and cash equivalents $ 168,215 $ 139,482 $ 86,671 Restricted cash, as part of other assets 1,509 1,509 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 169,724 $ 140,991 $ 87,133 The $1.5 million in restricted cash is comprised of $0.5 million in a letter of credit as a security deposit for the office and laboratory lease at Technology Square in Cambridge, Massachusetts and $1.0 million in a letter of credit as a security deposit for the Company’s office lease at Hampshire Street in Cambridge, Massachusetts. The Company has recorded cash held to secure these letters of credit as restricted cash in restricted cash and other assets on the consolidated balance sheet. The restricted cash is classified as non-current based on the related lease terms. Fair Value Measurements The FASB Codification Topic 820, Fair Value Measurements and Disclosures, Level 1 Quoted prices in active markets for identical assets or liabilities. Level 2 Observable inputs other than Level 1 prices, 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. The Company’s financial instruments as of December 31, 2020 and 2019 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of December 31, 2020 and December 31, 2019, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of December 31, 2020 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 163,264 $ 113,505 $ 49,759 $ — Marketable securities: Commercial paper 158,913 — 158,913 — Corporate notes 33,433 — 33,433 — U.S. government agency securities and treasuries 13,045 — 13,045 — Total $ 368,655 $ 113,505 $ 255,150 $ — Fair Value as of December 31, 2019 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 132,193 $ 124,419 $ 7,774 $ — Marketable securities: Commercial paper 96,963 — 96,963 — Corporate notes 140,642 — 140,642 — U.S. government agency securities and treasuries 4,000 — 4,000 — Total $ 373,798 $ 124,419 $ 249,379 $ — Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third-party pricing services or other market observable data. The Company measures its cash equivalents at fair value on a recurring basis, which approximates the net asset value per share. The Company classifies some of its cash equivalents within Level 1 of the fair value hierarchy because they are valued using observable inputs that reflect quoted prices for identical assets in active markets. The Company measures its marketable securities at fair value on a recurring basis and classifies those instruments and some cash equivalents within Level 2 of the fair value hierarchy. The pricing services used by management utilize industry standard valuation models, including both income and market-based approaches and observable market inputs to determine the fair value of marketable securities and those cash equivalents classified within Level 2 of the fair value hierarchy. As of December 31, 20 20 , the fair value of the long-term debt, payable in installments through November 18, 202 6 , approximated its carrying value due to the proximity of the issuance of the Tranche D Loan date to December 31, 20 20 ( See Note 13, Long-Term Debt ) . Amortization of Debt Discount and Issuance Costs Long-term debt is initially recorded at its allocated proceeds, net of discounts and deferred costs. Debt discount and issuance costs, consisting of legal and other fees directly related to the debt, are offset against initial carrying value of the debt and are amortized to interest expense over the estimated life of the debt based on the effective interest method. Liability Related to Sale of Future Royalties The Company treats the liability related to sale of future royalties as a debt financing, as the Company has significant continuing involvement in the generation of the cash flows, to be amortized to interest expense using the effective interest rate method over the life of the related royalty stream. The liability related to sale of future royalties and the related interest expense are based on the Company’s current estimates of future royalties expected to be paid over the life of the arrangement. The Company will periodically assess the expected royalty payments using a combination of internal projections and forecasts from external sources. To the extent the Company’s future estimates of royalty payments are greater or less than previous estimates or the estimated timing of such payments is materially different than its previous estimates, the Company will prospectively recognize related non-cash interest expense. For further discussion of the sale of future royalties, refer to Note 12, Sale of Future Royalties Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to its available cash, cash equivalents and marketable securities. The analysis for the year ended December 31, 2020 included consideration of the Company’s current cash needs, including its research and development plans, commercialization activities associated with the ongoing launch of TAZVERIK in the ES and FL indications, and its existing debt service obligations. The Company also evaluated its forecasted product revenues from sales of TAZVERIK. Such estimates of future sales contain significant judgement as TAZVERIK was recently launched and there is little or no history with which to base such estimates. The Company expects its available cash, cash equivalents and marketable securities will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the filing date of this Annual Report on Form 10-K with the SEC. As a result, the Company concluded that it did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. The Company’s current operating plan is based on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it expects. Accounts Receivable The Company extends credit to customers based on its evaluation of the customer’s financial condition. The Company records receivables for all billings when amounts are due under standard terms. Accounts receivable are stated at amounts due net of applicable prompt pay discounts and other contractual adjustments as well as an allowance for doubtful accounts. The Company assesses the need for an allowance for doubtful accounts by considering a number of factors, including the length of time trade accounts receivable are past due, the customer’s ability to pay its obligation and the condition of the general economy and the industry as a whole. The Company will write off accounts receivable when the Company determines that they are uncollectible. In general, the Company has experienced no significant collection issues with its customers. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer. Accounts receivable represent amounts due from customers and collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection. For a further discussion of concentration of credit risk see Note 3, Product Revenue, Net . Property and Equipment The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 3 - 6 years Computer and office equipment, and furniture 3 - 10 years Leasehold improvements 3 - 6 years or term of respective lease, if shorter Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations. Impairment of Long-Lived Assets The Company reviews long-lived assets to be held and used, including property and equipment and intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets or asset group may not be recoverable. Evaluation of recoverability is based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. No such impairments were recorded during 2020, 2019 or 2018. Income Taxes The Company records deferred income taxes to recognize the effect of temporary differences between tax and financial statement reporting. The Company calculates the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made. The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0 % likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense. Refer to Note 8 , Income Taxes , for additional information regarding the Company’s income taxes. Revenue Recognition – Collaboration Revenue Under ASC 606, Revenue from Contracts with Customers The Company has entered into collaboration and license agreements, which are within the scope of ASC 606, to discover, develop, manufacture and commercialize product candidates. The terms of these agreements typically contain multiple promises or obligations, which may include: (i) licenses, or options to obtain licenses, to compounds directed to specific targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed targets. Payments to the Company under these agreements may include non-refundable license fees, customer option exercise fees, payments for research activities, reimbursement of certain costs, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company first evaluates license and/or collaboration arrangements to determine whether the arrangement (or part of the arrangement) represents a collaborative arrangement pursuant to ASC Topic 808, Collaborative Arrangements, , which represent a collaborative relationship and not a customer relationship, Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses – If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promises, whether the value of the license is dependent on the unsatisfied promises, whether there are other vendors that could provide the remaining promises, and whether it is separately identifiable from the remaining promises. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services – The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under above. Reimbursements from the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options – The Company’s arrangements may provide a collaborator with the right to select a target for licensing either at the inception of the arrangement or within an initial pre-defined selection period, which may, in certain cases, include the right of the collaborator to extend the selection period. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires. Milestone Payments – At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Royalties – For arrangements that include sales-based royalties, including milestone payments based on a 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). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 11, Collaborations Revenue Recognition – Product Revenue The Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. For a further discussion of accounting for net product revenue see Note 3, Product Revenue, Net Research and Development Expenses Research and development expenses are expensed as incurred. Research and development expenses are comprised of costs incurred in providing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract research and development services, and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. External research and development expenses associated with the Company’s programs include clinical trial site costs, clinical manufacturing costs, costs incurred for consultants and other outside services, such as data management and statistical analysis support, and materials and supplies used in support of the clinical and preclinical programs. Internal costs of the Company’s clinical programs include salaries, stock-based compensation, and the portion of the Company’s facility costs allocated to research and development expense. When vendors billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those vendors, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history for related activities and the expected duration of the vendor service contract, where applicable. The Company generally accrues expenses related to research and development activities based on the services received and efforts expended pursuant to contracts with multiple contract research organizations that conduct and manage clinical trials, as well as other vendors that provide research and development services. Payments for these activities are based on the terms of the individual arrangements and may result in payment terms that differ from the pattern of costs incurred. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of subjects and the completion of clinical trial milestones. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from estimates, the Company would adjust the accrual or prepaid accordingly in future periods. Stock-Based Compensation The Company measures employee and non-employee stock-based compensation based on the grant date fair value of the stock-based compensation award. The Company grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant, based on observable marke

Product Revenue Net

Product Revenue Net	12 Months Ended
	Dec. 31, 2020
TAZVERIK [Member]
Concentration Risk [Line Items]
Product Revenue Net	3. Product Revenue, Net The Company sells TAZVERIK in the United States principally to a limited number of specialty pharmacies, which dispense the product directly to patients, and specialty distributors, which in turn sell the product to hospital pharmacies and community practice pharmacies (collectively, healthcare providers) for the treatment of patients. The specialty pharmacies and specialty distributors are referred to as the Company’s customers. Product revenue is recognized by the Company in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services when the customer obtains control of the product, which occurs at a point in time, typically when the product is received by the Company’s customers. The Company provides a right of return to its customers for unopened product for a limited time before and after its expiration date, which lapses upon shipment to a patient. Healthcare providers to whom specialty distributors sell TAZVERIK hold limited inventory that is designated for patients, and the Company monitors inventory levels in the distribution channel, to limit the risk of return. Reserves for Variable Consideration Revenues from product sales are recorded as product revenue at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established and which result from discounts, returns, chargebacks, rebates, co-pay assistance and other allowances that are offered within contracts between the Company and its customers, health care providers, payors and other indirect customers relating to the Company’s product sales. These reserves are based on the amounts earned or to be claimed on the related sales and are classified as reductions of accounts receivable (if the amount is payable to the customer) or a current liability (if the amount is payable to a party other than a customer). Where appropriate, these estimates take into consideration a range of possible outcomes that are probability-weighted for relevant factors such as the Company’s historical experience, current contractual and statutory requirements, specific known market events and trends, industry data and forecasted customer buying and payment patterns. Ove rall, these reserves reflect the Company’s best estimates of the amount of consideration to which the Company is entitled based on the terms of the contract (s) . The amount of variable consideration that is included in the transaction price may be constrained, and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. Actual amounts of consideration ultimate ly received may differ from the Company’s estimates. If actual res ults in the future vary from the Company’s estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such variances become known. Trade Discounts and Allowances : The Company generally provides customers with discounts that include incentive fees that are explicitly stated in customer contracts and are recorded as a reduction of revenue in the period the related product revenue is recognized. In addition, the Company receives sales order management, data and distribution services from certain customers. To the extent the services received are distinct from the Company’s sale of products to the customer, these payments are classified in selling, general and administrative expenses in the consolidated statements of operations and comprehensive loss. Product Returns : Consistent with industry practice, the Company generally offers customers a limited right of return based on the product’s expiration date for product that has been purchased from the Company, which lapses upon shipment to a patient. The Company estimates the amount of product sales that may be returned by customers and records this estimate as a reduction of revenue in the period in which the related product revenue is recognized. The Company currently estimates product return liabilities using available industry data and the Company’s own historical sales information, including its visibility into the product remaining in the distribution channel. Provider Chargebacks and Discounts : Chargebacks for fees and discounts to healthcare providers represent the estimated obligations resulting from contractual commitments to sell products to qualified healthcare providers at prices lower than the list prices charged to customers who directly purchase the product from the Company. Customers charge the Company for the difference between what they pay for the product and the ultimate selling price to the qualified healthcare providers. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and accounts receivable. Chargeback amounts are generally determined at the time of resale to the qualified healthcare provider by customers, and the Company generally issues credits for such amounts within a few weeks of the customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel at each reporting period end that the Company expects will be sold to qualified healthcare providers, and chargebacks that customers have claimed but for which the Company has not yet issued a credit. Government Rebates: The Company is subject to discount obligations under state Medicaid programs and Medicare. The Company estimates its Medicaid and Medicare rebates based upon a range of possible outcomes that are probability-weighted for the estimated payor mix. These reserves are recorded in the same period in which the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability that is included in accrued expenses on the Company’s consolidated balance sheet. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but remains in the distribution channel inventories at period end. Payor Rebates: The Company may contract with various private payor organizations, primarily insurance companies and pharmacy benefit managers, for the payment of rebates with respect to utilization of the Company’s products. The Company estimates these rebates and records such estimates in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability. Other Incentives/Patient Assistance Programs: The Company also offers voluntary patient assistance programs such as co-pay assistance. Co-pay assistance programs are intended to provide financial assistance to qualified commercially insured patients with prescription drug co-payments required by payors. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive associated with product that has been recognized as revenue, but remains in the distribution channel inventories at period end. The following table summarizes activity in each of the above product revenue allowances and reserve categories for the year ended December 31, 2020: Chargebacks, Discounts, and Government and Other Fees Rebates Returns Total (In thousands) Balance, January 1, 2020 $ — $ — $ — $ — Provision 802 1,046 67 1,915 Payments or credits (669 ) (618 ) — (1,287 ) Balance, December 31, 2020 $ 133 $ 428 $ 67 $ 628 Concentration of Credit Risk Financial instruments which potentially subject the Company to concentrations of credit risk consist of accounts receivable from customers and cash held at financial institutions. The Company believes that such customers and financial institutions are of high credit quality. For the years ended December 31, 2020 and 2019, net product revenue was primarily accounted from four individual customers. Revenue earned from each customer as a percentage of net product revenue is as follows: Year Ended December 31, 2020 2019 Customer 1 45 % — Customer 2 11 % — Customer 3 20 % — Customer 4 24 % — There was no product revenue for the year ended December 31, 2019. As of December 31, 2020, four individual customers accounted for a percentage of accounts receivable as follows: December 31, 2020 December 31, 2019 Customer 1 21 % — Customer 2 14 % — Customer 3 29 % — Customer 4 36 % — No other customer accounted for more than 10 percent of net product revenue or accounts receivable.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, net	4. Property and Equipment, net Property and equipment, net consists of the following: December 31, 2020 2019 (In thousands) Laboratory equipment $ 4,435 $ 4,273 Computer and office equipment, and furniture 4,636 5,113 Leasehold improvements 453 424 Construction in progress 34 560 Property and equipment 9,558 10,370 Less: accumulated depreciation and amortization (7,406 ) (8,151 ) Property and equipment, net $ 2,152 $ 2,219 Depreciation and amortization expense was $4.0 million, $0.8 million and $1.1 million for the years ended December 31, 2020, 2019, and 2018, respectively.

Inventory

Inventory	12 Months Ended
	Dec. 31, 2020
Inventory Disclosure [Abstract]
Inventory	5. Inventory All of the Company’s inventory relates to the manufacturing of TAZVERIK. The following table sets forth the Company’s inventory as of December 31, 2020 and December 31, 2019: December 31, 2020 December 31, 2019 (In thousands) Raw materials $ 1,068 $ — Work in process 8,564 — Finished goods 829 — Total $ 10,461 $ — As of December 31, 2020, the Company has not capitalized inventory costs related to its other drug development programs.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Prepaid Expenses and Other Current Assets	6. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following: December 31, 2020 2019 (In thousands) Prepaid clinical and manufacturing costs $ 12,646 $ 7,657 Interest receivable on available for sale securities 369 943 Other prepaid expenses and other receivables 4,906 6,923 Total prepaid expenses and other current assets $ 17,921 $ 15,523

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued Expenses	7. Accrued Expenses Accrued expenses consisted of the following: December 31, 2020 2019 (In thousands) Employee compensation and benefits $ 11,921 $ 7,844 Research and development expenses 10,664 9,706 Professional services and other 5,970 4,999 Accrued expenses $ 28,572 $ 22,565

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes The Company’s losses before income taxes consist solely of domestic losses. The provision for (benefit from) income taxes for the years ended December 31, 2020, 2019, and 2018 is as follows: 2020 2019 2018 (In thousands) Current $ 22 $ (34 ) $ (127 ) Deferred 92 92 184 Total 114 58 57 Income tax provision $ 114 $ 58 $ 57 A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 2018 Federal statutory income tax rate 21.0 % 21.0 % 21.0 % State income taxes 6.1 6.0 5.7 Research and development and other tax credits 2.0 1.9 2.4 Permanent items (0.6 ) (0.7 ) (0.7 ) Change in valuation allowance (27.9 ) (27.5 ) (28.3 ) Other (0.6 ) (0.7 ) (0.1 ) Effective income tax rate 0.0 % 0.0 % 0.0 % Deferred Tax Assets (Liabilities) The Company’s deferred tax assets (liabilities) included in other assets in the consolidated balance sheets consist of the following: December 31, 2020 2019 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 238,792 $ 180,877 Research and development and other credit carryforwards 34,205 29,774 Capitalized start-up costs 768 901 Deferred revenue — 1,029 Accruals and allowances 2,949 1,922 Eisai license payment 11,935 12,840 Stock compensation 7,338 6,489 Other 452 442 Sale of royalty 3,857 3,458 Lease liability 5,462 5,990 Business interest 1,220 — Gross deferred tax assets 306,978 243,722 Deferred tax asset valuation allowance (302,137 ) (237,858 ) Total deferred tax assets 4,841 5,864 Deferred tax liabilities: Depreciation and other (18 ) (40 ) Right of use asset (4,709 ) (5,732 ) Total deferred tax liabilities (4,727 ) (5,772 ) Net deferred tax asset (liability) $ 114 $ 92 The Company evaluated the expected recoverability of its net deferred tax assets as of December 31, 2020 and 2019, and determined that, with the exception of the deferred tax asset related to alternative minimum tax (“AMT”) credits, there was insufficient positive evidence to support the recoverability of these net deferred tax assets, concluding it is more likely than not that its net deferred tax assets would not be realized in the future; therefore, the Company provided a full valuation allowance against its net deferred tax asset balance as of December 31, 2020 and 2019, with the exception of the deferred tax asset related to the AMT credit. The AMT credit became refundable beginning in 2018 through no later than 2022 under the Tax Cuts and Jobs Act (“TCJA”), tax reform legislation, and as such, the related deferred tax asset will be able to be realized and the corresponding valuation allowance of $368,000 was reversed as of December 31, 2018 and recognized as a tax benefit. As of December 31, 2019, $184,000 of the deferred tax asset was reclassified to an income tax receivable. Fifty percent of the remaining AMT credit is refundable with the filing of the 2020 tax return. As such, as of December 31, 2020, $92,000 of the deferred tax asset was reclassified to an income tax receivable. There was no tax benefit or provision as a result of the asset reclassification on the consolidated balance sheet. Under the Coronavirus Aid, Relief, and Economic Security Act, or CARES, the AMT credit became 100% refundable with the filing of the Company’s tax return for the year ended December 31, 2019. The Company re-classed the remaining deferred tax asset to an income tax receivable as part of the 2019 provision to return analysis. At December 31, 2020 there is no deferred tax asset related to AMT credits and the Company has a full valuation allowance against its net deferred tax assets. As of December 31, 2020, the Company had operating loss carryforwards of approximately $877.8 million and $908.0 million available to offset future taxable income for United States federal and state income tax purposes, respectively. The U.S. federal tax operating loss carryforwards of $428.5 million will expire at various dates from 2029 through 2037. Approximately $449.2 million of the U.S. federal tax operating losses can be carried forward indefinitely. The state tax operating loss carryforwards expire commencing in 2030. Additionally, as of December 31, 2020 , the Company had research and development tax credit carryforwards of approximately $ 12.7 million and $ 4.8 million available to be used as a reduction of federal income taxes and state income taxes, respectively, which expire at various dates from 2028 through 20 40 , as well as federal orphan drug tax credit carryforwards of $ 17.7 million, which would expire at various dates from 2033 through 20 40 . The Company’s ability to use its operating loss carryforwards and tax credits to offset future taxable income is subject to restrictions under Section 382 of the U.S. Internal Revenue Code of 1986, as amended ( the “ Internal Revenue Code”). These restrictions may limit the future use of the operating loss carryforwards and tax credits if certain ownership changes described in the Internal Revenue Code occur. Future changes in stock ownership may occur that would create further limitations on the Company’s use of the operating loss carryforwards and tax credits. In such a situation, the Company may be required to pay income taxes, even though significant operating loss carryforwards and tax credits exist . Uncertain Tax Positions The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2020 2019 (In thousands) Unrecognized tax benefits - as of beginning of year $ 6,328 $ 5,743 Gross increases - current period tax positions 832 585 Unrecognized tax benefits - as of end of year $ 7,160 $ 6,328 None of the Company’s unrecognized tax benefits would result in income tax expense or impact the Company’s effective tax rate if recognized. The Company had no accrued tax-related interest or penalties as of December 31, 2020 or 2019. The Company has generated research and development and orphan drug credits, but has not conducted a study to document the qualified activities. This study may result in an adjustment to the Company’s reserve for uncertain tax positions, research and development credit, and orphan drug credit carryforwards. The Company files income tax returns in the U.S. federal tax jurisdiction and various state tax jurisdictions. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies Commitments In addition to commitments under leasing arrangements (Refer to Note 10, Leases the Company entered into a fourth amendment to the companion diagnostics agreement. Under the amended agreement, the Company and Roche Molecular agreed to divide a $1.0 million regulatory milestone for the United States into two separate milestone payments, of which $0.5 million was paid by the Company as part of the signed amendment, and the remaining $0.5 million was paid by the Company in December 2019 upon the satisfaction of certain conditions set forth in the fourth amendment to the companion diagnostics agreement. Through December 31, 2020, the Company has paid Roche Sequencing $9.4 million under the amended agreement, including developmental costs of $3.4 million paid in 2020, $4.0 million paid in 2019 and $2.0 million paid in 2018, respectively, upon the achievement of milestones under the amended agreement with Roche Sequencing. As of December 31, 2020, the Company is responsible for the remaining development costs of $1.0 million due under the agreement. Company paid $ 1.0 million to Roche Sequencing for the achievement of a development milestone in the fourth quarter of 2020. Additionally, the Company enters into contracts in the normal course of business with clinical research organizations for clinical and preclinical research studies, external manufacturers for product for use in clinical trials, and other research supplies and other services as part of the Company’s operations. These contracts generally provide for termination on notice, and therefore are cancelable contracts and not included in contractual commitments.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	10. Leases The Company enters into lease arrangements for its facilities as well as certain equipment. A summary of the arrangements are as follows: Operating Leases The Company leases office and laboratory space at Technology Square in Cambridge, Massachusetts under a Lease Agreement, dated as of June 15, 2012, as amended, or the Technology Square Lease, with ARE-TECH Square, LLC, a Delaware limited liability company. In May 2017, the Company exercised its option to extend the term of the Technology Square Lease to November 30, 2022. Under the Technology Square Lease as amended, the Company agreed to pay a monthly base rent of approximately $0.2 million for the period commencing December 1, 2017 through May 31, 2018, with an increase on June 1, 2018 of approximately $33,000 and annual increases of approximately $9,000 on December 1 of each subsequent year until the last increase, which will occur on December 1, 2021. Under the current terms of the Technology Square Lease, the Company does not have any further right to extend the term beyond November 30, 2022. The Company has a $0.5 million letter of credit as a security deposit for the Technology Square Lease and has recorded cash held to secure this letter of credit as restricted cash and other assets on the consolidated balance sheet. In applying the ASC 2016-02, Leases On August 16, 2019, the Company entered into a lease, or the Hampshire Street Lease, with BMR-Hampshire LLC, or BMR. The Hampshire Street Lease is for 33,525 rentable square feet of office space in Cambridge, Massachusetts. The Hampshire Street Lease commenced as of December 1, 2019. The Hampshire Street Lease has an initial term of seven years and four months from the commencement date and provides the Company with an option to extend the lease term for one additional five-year period. After a four-month period during which base rent was not payable, the Hampshire Street Lease provides for monthly rent payments starting at approximately $0.2 million and increasing 2.5% per year. In the event that the Company exercises its option to extend the lease term, the Hampshire Street Lease provides for monthly rent payments during the additional five-year period at the greater of the base rent rate at the end of the initial term or the then-current market rent. The Company has a $1.0 million letter of credit in favor of BMR as a security deposit for the Hampshire Street Lease and has recorded cash held to secure this letter of credit as restricted cash and other assets on the consolidated balance sheet. In applying ASC 842, the Company determined the classification of the Hampshire Street Lease to be operating and recorded a lease liability and a right-of-use asset as of December 31, 2019. The Company is required to pay certain variable costs to its landlords in addition to fixed rent. These costs include common area maintenance, real estate taxes, and parking and are included in lease expense. The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the years ended December 31, 2020 and 2019: Twelve months ended December 31, Twelve months ended December 31, 2020 2019 Lease cost Operating lease cost $ 6,155 $ 3,771 Variable lease cost 1,764 1,318 Total lease cost $ 7,919 $ 5,089 Other information Operating cash flows used for operating leases $ 4,374 $ 3,648 Weighted average remaining lease term 5.3 years 5.3 years Weighted average discount rate 9.77 % 9.60 % Rent expense was $3.5 million for the year ended December 31, 2018 under the previous guidance in ASC 840, Leases Future minimum lease payments under the Company’s non-cancelable operating leases as of December 31, 2020 and 2019, are as follows: 2020 (In thousands) 2021 $ 6,436 2022 6,256 2023 2,984 2024 3,053 Thereafter 6,966 Total lease payments $ 25,695 Less: imputed interest (5,621 ) Total operating lease liabilities at December 31, 2020 $ 20,074

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	11. Collaborations GSK In January 2011, the Company entered into a collaboration and license agreement with Glaxo Group Limited (an affiliate of GlaxoSmithKline plc), or GSK, to discover, develop and commercialize novel small molecule HMT inhibitors directed to available targets from the Company’s platform. Under the terms of the agreement, the Company granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration, the Company agreed to provide research and development services related to the licensed targets pursuant to agreed upon research plans during a research term that ended January 8, 2015. In March 2014, the Company and GSK amended certain terms of this agreement for the third licensed target, revising the license terms with respect to candidate compounds and amending the corresponding financial terms, including reallocating milestone payments and increasing royalty rates as to the third target. Subsequent to a GSK strategic portfolio prioritization, the Company received notice in October 2017 that GSK terminated the agreement with respect to the third target, effective December 31, 2017, which returned all rights to that target to the Company. The two other targets, PRMT5 and PRMT1, continue to be subject to the agreement and were not impacted by the termination with respect to the third target. The Company substantially completed all research obligations under this agreement by the end of the first quarter of 2015 and completed the transfer of the remaining data and materials for these programs to GSK in the second quarter of 2015. Agreement Structure Under the agreement, the Company has received and recognized as collaboration revenue totaling $89.0 million, consisting of upfront payments, fixed research funding, research and development services and preclinical and research and development milestone payments. As of December 31, 2020, for the two remaining targets, the Company is eligible to receive up to $50.0 million in clinical development milestone payments, up to $197.0 million in regulatory milestone payments and up to $128.0 million in sales-based milestone payments. As a result of the termination of the agreement as it relates to the third target, the Company will receive no Collaboration Revenue Through December 31, 2020, the Company has earned a total of $89.0 million in total collaboration revenue since inception of the GSK agreement, which the Company recognized as collaboration revenue in the consolidated statements of operations and comprehensive loss, including $20.0 million in milestone revenue in the year ended December 31, 2018. The Company did not recognize any collaboration revenue under the agreement in the years ended December 31, 2020 and 2019. The Company did not have any deferred revenue related to this agreement as of December 31, 2020 or December 31, 2019 and any future revenues will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets. All remaining milestone payments as of December 31, 2020 have been deemed not probable and therefore have not been recognized as revenue. Eisai In April 2011, the Company entered into a collaboration and license agreement with Eisai, under which the Company granted Eisai an exclusive worldwide license to its small molecule HMT inhibitors directed to the EZH2 HMT, including the Company’s product candidate tazemetostat, while retaining an opt-in right to co-develop, co-commercialize and share profits with Eisai as to licensed products in the United States. As of December 31, 2014, the Company had completed its performance obligations under the original agreement. In March 2015, the Company entered into an amended and restated collaboration and license agreement with Eisai (the “Eisai License Agreement”), under which the Company reacquired worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat. Under the Eisai License Agreement, the Company is responsible for global development, manufacturing and commercialization outside of Japan of tazemetostat and any other EZH2 product candidates, with Eisai retaining development and commercialization rights in Japan, as well as a right to elect to manufacture tazemetostat and any other EZH2 product candidates in Japan, and a right of first negotiation for the rest of Asia. Eisai waived its right of first negotiation for the rest of Asia in 2018. Under the original collaboration and license agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the Eisai License Agreement, the Company is solely responsible for funding global development, manufacturing and commercialization costs for EZH2 compounds outside of Japan, including the remaining development costs due under a companion diagnostics agreement with Roche Molecular Systems, Inc., or Roche Molecular, which was amended to assign all of Roche Molecular’s rights and obligations under the companion diagnostics agreement to Roche Sequencing, effective January 1, 2020. Eisai is solely responsible for funding Japan-specific development and commercialization costs for EZH2 compounds. The Company recorded the reacquisition of worldwide rights, excluding Japan, to the EZH2 program, including tazemetostat, under the Eisai License Agreement, as an acquisition of an in-process research and development asset. As this asset was acquired without corresponding processes or activities that would constitute a business, had not achieved regulatory approval for marketing and, absent obtaining such approval, had no alternative future use, the Company recorded the $40.0 million upfront payment made to Eisai in March 2015 as research and development expense in the consolidated statements of operations and comprehensive loss. The C ompany also agreed to pay Eisai up to $ 20.0 million in clinical development milestone s , and up to $ 50.0 million in regulatory milestone payments, and royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan. The Company is eligible to receive from Eisai royalties at a percentage in the mid-teens on net sales of any EZH2 product in Japan . In the second quarter of 2019, the Company submitted its first NDA to the FDA, for the treatment of patients with ES, triggering the payment of the first $10.0 million clinical development milestone to Eisai and the recording of this amount to research and development expense. The Company paid the $10.0 million clinical development milestone to Eisai in June 2019. In the fourth quarter of 2019, the Company submitted its second NDA to the FDA, for the treatment of patients with FL, triggering the payment of the second $10.0 million clinical development milestone to Eisai and the recording of this amount to research and development expense. The Company paid the $10.0 million clinical development milestone to Eisai in December 2019. In January 2020, the Company triggered the payment of the $25.0 million milestone payment upon regulatory approval of tazemetostat for ES, which was capitalized as an intangible asset on the Company’s consolidated balance sheet as of December 31, 2020. In June 2020, the Company triggered the payment of the $25.0 million milestone payment upon regulatory approval of tazemetostat for FL, which was capitalized as an intangible asset on the Company’s consolidated balance sheet as of December 31, 2020. Sale of Future Royalties Roche In December 2012, Eisai and the Company entered into a companion diagnostics agreement with Roche Molecular, under which Eisai and the Company engaged Roche Molecular to develop a companion diagnostic to identify patients who possess certain activating mutations of EZH2. In October 2013, this agreement was amended to include additional mutations in EZH2. The development costs due under the amended agreement with Roche Molecular were the responsibility of Eisai until the execution of the amended and restated collaboration and license agreement with Eisai in March 2015, at which time the Company assumed responsibility for the remaining development costs due under the agreement. In December 2015, the Company and Eisai entered into a second amendment to the companion diagnostics agreement with Roche Molecular. The agreement was further amended in March 2018. Under the amended agreement, the Company was responsible for remaining development costs of $10.4 million due under the agreement as of March 2018 and Eisai agreed to reimburse the Company $0.9 million of this amount related to a regulatory milestone for Japan. In July 2019, the Company entered into a fourth amendment to the companion diagnostics agreement. Under the amended agreement, the Company and Roche Molecular agreed to divide a $1.0 million regulatory milestone for the United States into two separate milestone payments, of which $0.5 million was paid by the Company as part of the signed amendment, and the remaining $0.5 million was paid by the Company in December 2019 upon the satisfaction of certain conditions set forth in the fourth amendment to the companion diagnostics agreement. Under the agreement with Roche Sequencing , Roche Sequencing is obligated to use commercially reasonable efforts to develop and to make commercially available the companion diagnostic. Roche Sequencing has exclusive rights to commercialize the companion diagnostic. On June 18, 2020 the FDA approved the companion diagnostic that is intended to identify follicular lymphoma patients with an EZH2 mutation for treatment with tazemetostat. The agreement with Roche Sequencing will expire when the Company and Eisai are no longer developing or commercializing tazemetostat. The Company and Eisai may terminate the agreement by giving Roche Sequencing 90 days’ written notice if the Company and Eisai discontinue development and commercialization of tazemetostat or determine, in conjunction with Roche Sequencing, that the companion diagnostic is not needed for use with tazemetostat. Any party may also terminate the agreement in the event of a material breach by any party, in the event of material changes in circumstances that are contrary to key assumptions specified in the agreement or in the event of specified bankruptcy or similar circumstances. Under specified termination circumstances, Roche Sequencing may become entitled to specified termination fees. Boehringer Ingelheim In November 2018, the Company entered into a collaboration and license agreement with Boehringer Ingelheim International GmbH (“Boehringer Ingelheim”) to discover, research, develop and commercialize small molecule compounds that are inhibitors of an undisclosed histone acetyltransferase, or HAT, target and an undisclosed helicase target, along with associated predictive biomarkers (the “Target Projects”). Under the terms of the agreement, the Company granted to Boehringer Ingelheim an exclusive, worldwide license to the undisclosed target inhibitors technology. The agreement also included reciprocal licenses to utilize each other’s know-how, patents and technologies for activities under the agreement. Further, each party was granted the license to develop, manufacture, commercialize and otherwise exploit any compound or product that successfully achieves start of lead optimization (“SoLO”). The Company was also obligated to provide R&D services through SoLO approval for both Target Projects, and to serve on the Joint Steering Committee (“JSC”) throughout without cause, and in accordance with the terms of the Collaboration Agreement and the parties’ agreement. The termination became effective on January 31, 2021. The Target Project for the helicase target and the reciprocal licenses terminated as of this date. The Company is entitled to pursue the HAT target and helicase target programs in all fields worldwide without further obligation to . Agreement Structure Under the terms of the agreement, the Company received a $15.0 million upfront payment and $5.0 million in research funding for the costs to be incurred by the Company in connection with its research activities, payable quarterly in four equal installments during 2019. At its discretion, Boehringer Ingelheim had the option to extend the research period by up to one year, subject to the Company’s agreement to the specified research activities and additional research funding. During the third quarter of 2019, Boehringer Ingelheim’s option to extend the research period expired unexercised, and therefore the research period ended on December 31, 2019. In March 2020, the Company and Boehringer Ingelheim amended the agreement to extend the research period for the shared program targeting enzymes within helicase families with Boehringer Ingelheim providing research funding of $0.4 million. Additionally, in March 2020, the Company received notice of termination for the program targeting enzymes with HAT families, which program termination became effective in June 2020. In September 2020, the Company and Boehringer Ingelheim further amended the agreement to extend the research period for the shared program targeting enzymes within helicase families with Boehringer Ingelheim to provide research funding of $0.1 million. The additional research activities were completed prior to the end of 2020. Accounting Considerations of the Agreement The Company assessed the arrangement in accordance with ASC 606 and concluded that the contract counterparty, Boehringer Ingelheim, is a customer based on the arrangement structure, through the satisfaction of each target’s performance obligations. The Company identified the following performance obligations under the arrangement: • the combination of the Epizyme license to the first undisclosed target inhibitor technology, associated research and development services through the research period and, • the combination of the Epizyme license to the second undisclosed target inhibitor technology, associated research and development services through the research period. The Company determined that each Epizyme license was not distinct from the associated research and development services due to the limited economic benefit that Boehringer Ingelheim would derive from the Epizyme license if the research services were not provided by the Company. Accordingly, the Epizyme licenses and associated research and development services, for each Target Project, are each accounted for as a combined performance obligation. Under the agreement, the Company determined that the total transaction price at execution was $20.0 million, comprised of the following: • $15.0 million total upfront payment received under the agreement; and • $5.0 million research funding payment to be received in 2019. In addition, during 2019, the Company achieved a $5.5 million development milestone for selection of a lead optimization candidate for the shared program targeting enzymes within helicase families, which was added to the transaction price. The future potential milestone payments were excluded from the transaction price at inception, as the achievement of the milestone events are highly uncertain. As such, all milestone payments were fully constrained. The Company reevaluated the transaction price at the end of each reporting period and as uncertain events were resolved or other changes in circumstances occurred, it adjusted its estimate of the transaction price. The Company recognized a total of $23.8 million related to the two performance obligations through December 31, 2019. During the year-ended 2020, the Company added $0.5 million in research funding for the shared program targeting enzymes within helicase families to the transaction price was recognized as the research performance obligation was performed prior to the end of 2020. Collaboration Revenue Through December 31, 2020, the Company has recognized $26.0 million in total collaboration revenue since the inception of this collaboration, including $0.5 million during the year ended December 31, 2020. The Company recognized $23.8 million and $1.7 million of revenue as of December 31, 2019 and 2018, respectively. As of December 31, 2020 and 2019, the Company did not have any deferred revenue related to this agreement. As of December 31, 2020 and 2019, the Company had accounts receivable of $0.0 million and $1.3 million, respectively. Celgene (a subsidiary of Bristol-Myers Squibb Company) In April 2012, the Company entered into a collaboration and license agreement with Celgene Corporation, or Celgene. On July 8, 2015, the Company entered into an amendment and restatement of the collaboration and license agreement with Celgene, or the Celgene Collaboration Agreement. All performance obligations, except for the three material rights were substantially satisfied as of the adoption of ASC 606 and therefore all of the transaction price allocated to those performance obligations has been recognized as revenue under ASC 606. Through December 31, 2020, the Company has recognized revenue of $99.2 million under the agreement as collaboration revenue in the Company’s consolidated statements of operations and comprehensive loss and in accumulated deficit as a result of the cumulative-effect recognition upon adoption of ASC 606. The amounts received that ha d previously not been recognized as revenue and were recorded in deferred revenue on the Company’s consolidated balance sheet , relate to the material rights On November 3, 2020, the Company received a notice of termination without cause of the Celgene Collaboration Agreement, and such termination became effective on January 2, 2021. As a result, deferred revenue related to the agreement of $3.8 million was recognized as revenue in the quarter ended December 31, 2020.

Sale of Future Royalties

Sale of Future Royalties	12 Months Ended
	Dec. 31, 2020
Deferred Revenue Disclosure [Abstract]
Sale of Future Royalties	12. Sale of Future Royalties On November 4, 2019, the Company entered into a loan agreement with BioPharma Credit PLC, or the Collateral Agent, and the Lenders On November 18, 2020, we borrowed the Tranche D Loan Long-Term Debt the Amended and Restated Loan Agreement the Company has the right to request up to an additional $150.0 million in secured term loans, subject to the approval of the Lenders, provided that the Company has not prepaid any outstanding term loans at the time of the Company’s request and such request is made before November 18, 2021. On November 4, 2019, the Company also executed a purchase agreement (the “RPI Purchase Agreement”) with RPI. Pursuant to the RPI Purchase Agreement, the Company agreed to sell to RPI 6,666,667 shares of its common stock, a warrant to purchase up to 2,500,000 shares of common stock at an exercise price of $20.00 per share (the “Common Stock Warrant”), and all of the Company’s rights to receive royalties from Eisai with respect to net sales by Eisai of tazemetostat products in Japan pursuant to the Eisai License Agreement and any successor arrangement for Japan sales (the “Japan Royalty”, and collectively, the “Transaction”). In consideration for the sale of shares of common stock, the Common Stock Warrant and the Japan Royalty, RPI paid the Company $100.0 million upon the closing of the RPI Purchase Agreement. In addition, RPI agreed, in connection with RPI’s acquisition from Eisai of the right to receive royalties from the Company under the Eisai License Agreement, to reduce the Company’s royalty obligation by low single digits upon the achievement of specified annual net sales levels over $1.5 billion. In addition, under the RPI Purchase Agreement, the Company had the right to sell, and RPI has the obligation to purchase, subject to certain conditions, including a maximum purchase price of $20.00 per share, $50.0 million of shares of common stock at the Company’s option for an 18-month period from the date of execution of the RPI Purchase Agreement (the “Put Option”). In February 2020, the Company sold 2.5 million shares of its common stock to RPI, for an aggregate of $50.0 million in proceeds pursuant to the Put Option. Additionally, under the terms of the RPI Purchase Agreement, the founder and chief executive officer of RP Management, an affiliate of RPI, and a co-founder of Pharmakon Advisors LP, an affiliate of the Lenders was elected as a director of the Company. As of December 31, 2020 and 2019, RPI and its affiliates owned 9.0% and 6.8% of the Company’s common stock, respectively. The Company accounted for the Loan Agreement and RPI Purchase Agreement as a single arrangement as RPI and the Lenders are related parties and the agreements were negotiated together. The aggregate proceeds of $125.0 million were allocated on a relative fair value basis, which approximated their respective actual fair values, to the four units of accounting pursuant to the transaction as follows: (1) $79.0 million to the common stock issued to RPI based on the closing price of the Company’s common stock on the date of the transaction, (2) $8.4 million to the Common Stock Warrant to purchase shares of common stock, based on the Black-Scholes option pricing model, (3) $12.6 million to the liability related to the sale of future royalties based on a discounted cash flow model and (4) $25.0 million to the Tranche A Note Payable based on the terms of the Loan Agreement. Transaction costs of $2.0 million were allocated directly to the units of accounting it relates to. The fair value for the liability related to the sale of future royalties at the time of the execution of the transaction was based on estimates of future royalties expected to be paid to RPI over the life of the arrangement, which are considered level 3 inputs. The allocated fair value of the common stock and Common Stock Warrant have been recorded in additional paid-in-capital and the Tranche A Note Payable has been recorded as long-term debt (See Note 13, Long-Term Debt Although the Company sold all of its rights to receive the Japan Royalty, under the terms of the RPI Agreement, the Company continues to own all tazemetostat intellectual property rights and is responsible for any ongoing manufacturing and supply obligations agreed to by the Company and Eisai pursuant to the Eisai License Agreement related to the generation of these royalties. Due to the Company’s continuing involvement, the Company will continue to account for any royalties due as revenue and recorded the proceeds from this transaction as a liability (“Royalty Obligation”) that will be accreted using the effective interest method over the estimated life of the RPI Purchase Agreement. As royalties are remitted to RPI from Eisai, the balance of the Royalty Obligation will be effectively repaid over the life of the Eisai License Agreement. In order to determine the accretion of the Royalty Obligation, the Company is required to estimate the total amount of future royalty payments to RPI over the life of the Eisai License Agreement. The $12.6 million recorded at execution will be accreted to the total of these royalty payments as interest expense over the life of the Royalty Obligation. At execution, the Company’s estimate of this total interest expense resulted in an effective annual interest rate of approximately 9.01%. This estimate contains significant assumptions that impact both the amount recorded at execution and the interest expense that will be recognized over the royalty period. The Company periodically assesses the estimated royalty payments to RPI from Eisai and to the extent the amount or timing of such payments is materially different than the original estimates, an adjustment will be recorded prospectively to increase or decrease interest expense. There are a number of factors that could materially affect the amount and timing of royalty payments to RPI from Eisai, and correspondingly, the amount of interest expense recorded by the Company, most of which are not within the Company’s control. Such factors include, but are not limited to, delays or discontinuation of development of tazemetostat in Japan, regulatory approval, changing standards of care, the introduction of competing products, manufacturing or other delays, generic competition, intellectual property matters, 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 RPI are made in U.S. dollars (USD) while the underlying Japan sales of tazemetostat will be made in currencies other than USD, and other events or circumstances that are not currently foreseen as tazemetostat is still under development in Japan and subject to regulatory approval. Changes to any of these factors could result in increases or decreases to both royalty revenues and interest expense. As of December 31, 2020, the Company’s assessment of the estimated future royalty payments to RPI resulted in a current effective interest rate of approximately 13.4%. The following table shows the activity of the Royalty Obligation since the transaction inception through December 31, 2020: Year Ended December 31, 2020 (In thousands ) Proceeds from sale of future royalties $ 12,601 Non-cash interest expense recognized 1,575 Liability related to the sale of future royalties - ending balance $ 14,176 During the years ended December 31, 2020 and 2019 no non-cash royalties from net sales of tazemetostat in Japan were recorded and the Company recorded $1.4 million and $0.2 million, respectively, of related non-cash interest expense.

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Long-Term Debt	13. Long-Term Debt On November 4, 2019, the Company entered into the Loan Agreement, which provided for up to $70.0 million in secured term loans to be advanced in up to three tranches. The Company borrowed $70.0 million in the aggregate under the three tranches pursuant to the Loan Agreement. With the FDA’s June 2020 approval of tazemetostat for the treatment of FL in the United States, the Company also had the right, but not the obligation, to request up to an additional $300.0 million in secured term loans, subject to the approval of the Lenders, provided the Company has not prepaid any outstanding term loans at the time of such request and such request is made before November 18, 2021. On November 3, 2020, the Company entered into the Amended and Restated Loan Agreement with the Lenders. The Amended and Restated Loan Agreement provides for, among other things, an additional secured term loan of $150.0 million, or the Tranche D Loan. On November 3, 2020, the Company also delivered written notice to the Lenders to draw down the Tranche D Loan, which was funded on November 18, 2020. The Company paid a commitment fee of 2.00% of the origial $70.0 million committed facility amount in November 2019 and 2% of the $150.0 million Tranch D Loan in November 2020, as well as expenses incurred by the Lender in executing the agreements. The interest rate for the Tranche D Loan will be determined by reference to a Eurodollar rate plus 7.75% above such Eurodollar rate. The Eurodollar rate will have a 2.00% floor. The Tranche D Loan will be due in eight equal quarterly principal payments commencing on the 51st month anniversary of the date on which the Lenders fund the Tranche D Loan. All unpaid principal and interest under the Tranche D Loan will be due and payable on the 72nd month anniversary of the date on which the Lenders funded the Tranche D Loan. The Amended and Restated Loan Agreement also amended the payment period principal and interest for the first three tranches of term loans. Under the original terms, the Company was required to make interest only payments on the outstanding obligation through February 28, 2023, and thereafter eight quarterly payments of principal and interest. Under the amended and restated terms, the Company is required to make interest only payments on the $70.0 million outstanding obligation through November 2023, and thereafter four quarterly payments of principal and interest. All unpaid principal and interest on the $70.0 million borrowed under the original Loan Agreement is due and payable in November 2024, the 60th month anniversary of the date on which the Lenders funded the first tranche of term loans. The interest rates for the existing tranches of term loans remain unchanged and will continue to be determined by reference to a Eurodollar rate plus 7.75% above such Eurodollar rate. The Eurodollar rate will have a 2.00% floor. Under the Amended and Restated Loan Agreement the Company has the right to request from the Lenders, subject to the Lenders’ agreement to lend additional amounts to the Company, up to an additional $150.0 million, provided that the Company has not prepaid any outstanding term loans at the time of the Company’s request and such request is made before November 18, 2021. Each of the four term loans may be prepaid before maturity in whole or in part, however there is a $50.0 million minimum prepayment for any prepayment of the loans. If the Company prepays any tranche of term loans, in whole or in part, during the first 36 months from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to the greater of (x) a make-whole amount equal to the interest that would have accrued on the principal amount to be prepaid and (y) a premium equal to 0.03 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loan, in whole or in part, between the 36th month and 48th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.02 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loans, in whole or in part, between the 48th month and 60th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.01 multiplied by the principal amount to be prepaid. The Amended and Restated Loan Agreement was accounted for as a debt modification based on a comparison of the present value of the cash flows under the terms of the debt immediately before and after the effective date of the The Amended and Restated Loan Agreement, which resulted in a change of less than 10%. As a result, issuance costs paid to the Lenders in connection with the The Amended and Restated Loan Agreement were recorded as a reduction of the carrying amount of the debt liability and unamortized issuance costs as of the date of the modification are amortized to interest expense over the repayment term of The Amended and Restated Loan Agreement. The obligations under the Amended and Restated Loan Agreement, including the Company’s payment obligations in respect of the Tranche D Loan are secured by the first priority security interest in and a lien on substantially all of the assets of the Company, subject to certain exceptions, that the Company granted to the Lenders in connection with the first tranche of term loans under the Loan Agreement. The Amended and Restated Loan Agreement contains certain customary representations and warranties, affirmative and negative covenants and events of default applicable to the Company and its subsidiaries. If an event of default occurs and is continuing, the Collateral Agent may, among other things, accelerate the loans and foreclose on the collateral. The Company has determined that the risk of subjective acceleration under the material adverse events clause is not probable and therefore has classified the outstanding principal in non-current liabilities based on scheduled principal payments. The Company has the following minimum aggregate future loan payments at December 31, 2020 (in thousands): Year Ended December 31, 2020 2021 $ — 2022 — 2023 — 2024 70,000 2025 75,000 2026 75,000 Total minimum payments 220,000 Less amounts representing interest and discount (4,330 ) Less current portion — Long-term debt, net of current portion $ 215,670 For the years ended December 31, 2020 and 2019, interest expense related to the Company's Loan Agreement was approximately $7.2 million and $0.3 million, respectively. The total carrying value of debt is classified as long-term on the consolidated balance sheet as of December 31, 2020 and 2019, respectively.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Stockholders' (Deficit) Equity	14. Stockholders’ (Deficit) Equity Common Stock On March 24, 2020, the Company’s board of directors adopted, subject to stockholder approval, an amendment to the Company’s Restated Certificate of Incorporation to increase the number of authorized shares of common stock, $0.0001 par value per share, from 125,000,000 to 150,000,000 (the “Charter Amendment”). At the Company’s 2020 Annual Meeting of Stockholders, the stockholders of the Company approved the Charter Amendment, which was filed with the Secretary of State of the State of Delaware on May 29, 2020. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors. In February 2020, the Company sold 2,500,000 shares of its common stock in connection with the exercise of its Put Option to sell shares of its common stock for an aggregate of $49.9 million in net proceeds after deducting financing costs of $0.1 million. In March 2019, the Company issued 11,500,000 shares of Common Stock in connection with a public offering. Sale of Future Royalties The issuance of these shares contributed to significant increases in the Company’s shares of common stock outstanding as of December 31, 2020 and 2019 and in the weighted average shares outstanding for the years ended December 31, 2020 and 2019 when compared to the comparable prior year periods. As of December 31, 2020, a total of 20,405,623 shares of common stock were reserved for issuance upon (i) the exercise of outstanding stock options and the vesting of outstanding restricted stock units (ii) the issuance of additional stock awards under the Company’s 2013 Stock Incentive Plan and 2013 Employee Stock Purchase Plan (iii) the issuance of common stock upon conversion of the outstanding Series A Preferred Stock and (iv) the issuance of common stock under the warrants. Convertible Preferred Stock On March 6, 2019, the Company entered into an Underwriting Agreement, (the “Preferred Stock Agreement”), that related to the public offering of 350,000 shares of Series A Convertible Preferred Stock, par value $0.0001 per share (“Series A Preferred Stock”), for a purchase price to the public of $115.00 per share. All of the Series A Preferred Stock was sold by the Company for net proceeds of $37.4 million. Upon issuance, each share of Series A Preferred Stock included an embedded beneficial conversion feature because the market price of the Company’s common stock on the date of issuance of the Series A Preferred Stock at $12.34 per share as compared to an effective conversion price of the Series A Preferred Stock of $11.50 per share. As a result, the Company recorded the intrinsic value of the beneficial conversion feature of $2.9 million as a discount on the Series A Preferred Stock at issuance. Because the Series A Preferred Stock is immediately convertible upon issuance and does not include mandatory redemption provisions, the discount on the Series A Preferred Stock was immediately accreted. The Company evaluated the Series A Preferred Stock for liability or equity classification in accordance with the provisions of ASC 480, Distinguishing Liabilities from Equity Voting Rights Shares of Series A Preferred Stock will generally have no voting rights except as required by law and except that the consent of the holders of a majority of our outstanding shares of Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock or take certain other actions with respect to the Series A Preferred Stock. Dividends Shares of Series A Preferred Stock will be entitled to receive dividends equal to (on an as-if-converted-to-common stock basis), and in the same form and manner as, dividends actually paid on shares of the Company’s common stock. Liquidation Rights Subject to the prior and superior rights of the holders of any senior securities of the Company, upon liquidation, dissolution or winding up of the Company, whether voluntary or involuntary, each holder of shares of Series A Preferred Stock shall be entitled to receive, in preference to any distributions of any of the assets or surplus funds of the Company to the holders of common stock, an amount equal to $0.001 per share of Series A Preferred Stock, plus an additional amount equal to any dividends declared but unpaid on such shares, before any payments shall be made or any assets distributed to holders of any class of common stock. If, upon any such liquidation, dissolution or winding up of the Company, the assets of the Company shall be insufficient to pay the holders of shares of the Series A Preferred Stock the amount required under the preceding sentence, then all remaining assets of the Company shall be distributed ratably to holders of the shares of the Series A Preferred Stock in proportion to the respective amounts which would otherwise be payable in respect of the shares held by them upon such distribution if all amounts payable on or with respect to such shares were paid in full. Conversion Each share of Series A Preferred Stock shall be convertible, at any time and from time to time from and after the issuance date, at the option of the holder thereof, into a number of shares of common stock equal to 10 shares of common stock, provided that the holder will be prohibited from converting Series A Preferred Stock into shares of the Company’s common stock if, as a result of such conversion, the holder, together with its affiliates and attribution parties, would own more than 9.99% of the total number of shares of common stock then issued and outstanding. The holder can change this requirement to a higher or lower percentage, not to exceed 9.99% of the number of shares of common stock outstanding, upon 61 days’ notice to the Company. In February 2020, 12,200 shares of Series A Preferred Stock were converted to 122,000 shares of common stock. Redemption The Company is not obligated to redeem or repurchase any shares of Series A Preferred Stock. Shares of Series A Preferred Stock are not entitled to any redemption rights or mandatory sinking fund or analogous fund provisions. Warrants In November 2019, the Company issued the Common Stock Warrant for the purchase of up to 2,500,000 shares of Common Stock at an exercise price of $20.00 per share to RPI pursuant to the RPI Purchase Agreement (for additional information refer to Note 12, Sale of Future Royalties The Common Stock Warrant remain outstanding as of December 31, 2020.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Employee Benefit Plans	15. Employee Benefit Plans Stock Incentive Plans In 2008, the Company’s board of directors adopted and the Company’s stockholders approved the 2008 Stock Incentive Plan (the “2008 Plan”), which provided for the granting of certain defined stock incentive awards to employees, members of the Company’s board of directors and non-employee consultants, advisors or other service providers. In April 2013, the Company’s board of directors adopted and the Company’s stockholders approved the 2013 Stock Incentive Plan (the “2013 Plan”), which provides for the granting of certain defined stock incentive awards to employees, members of the Company’s board of directors and non-employee consultants, advisors or other service providers. Additionally, in May 2013, the Company’s board of directors adopted and the Company’s stockholders approved the 2013 Employee Stock Purchase Plan (the “2013 ESPP”), which provides participating employees the option to purchase shares of the Company’s common stock at defined purchase prices over six month offering periods. Stock incentive awards granted under the 2013 Plan may be incentive stock options, non-qualified stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards under the applicable provisions of the Internal Revenue Code. Incentive stock options are granted only to employees of the Company. Non-qualified stock options and restricted stock and restricted stock units may be granted to officers, employees, consultants, advisors and other service providers. Incentive and non-qualified stock options granted to employees and generally vest over four years, with 25.0% vesting upon the one-year anniversary of the grant and the remaining 75.0% vesting monthly over the following three years. Restricted stock units granted to employees generally vest over four years in equal annual installments. Non-qualified stock options granted to consultants and other non-employees generally vest over the period of service to the Company. Initial non-qualified stock options granted to members of the Company’s board of directors generally vest over four years, with 25% vesting upon the one-year anniversary of grant and the remaining 75% vesting monthly over the following three years. Annual non-qualified stock options granted to members of the Company’s board of directors vest on the one-year anniversary of the grant. Incentive and non-qualified stock options expire ten years from the date of grant. Stock-Based Compensation Total stock-based compensation expense related to stock options, restricted stock units, shares issued under the employee stock purchase plan, and shares granted to non-employee directors in lieu of board fees was $27.6 million, $18.0 million, and $12.0 million for the years ended December 31, 2020, 2019, and 2018, respectively. Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2020 2019 2018 (In thousands) Research and development $ 9,093 $ 6,295 $ 4,083 General and administrative 18,516 11,721 7,921 Total $ 27,609 $ 18,016 $ 12,004 Stock Options The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. Weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 0.9 % 2.2 % 2.6 % Expected life of options 5.99 years 6.0 years 6.0 years Expected volatility of underlying stock 70.9 % 72.0 % 71.5 % Expected dividend yield 0.0 % 0.0 % 0.0 % There were no stock option awards granted to non-director, non-employees in the years ended December 31, 2020, December 31, 2019 or 2018. The risk-free interest rate is based upon the U.S. Treasury yield curve in effect at the time of grant, with a term that approximates the expected life of the option. The Company calculates the expected life of options granted to employees using the simplified method as the Company has insufficient historical information to provide a basis for estimate. The Company determines the expected volatility using a blended approach encompassing its historical experience and the historical volatility of a peer group of comparable publicly traded companies with product candidates in similar stages of development to the Company’s product candidates. The Company has applied an expected dividend yield of 0.0% as the Company has not historically declared a dividend and does not anticipate declaring a dividend during the expected life of the options. The following is a summary of stock option activity for the year ended December 31, 2020: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2019 8,087 $ 12.86 Granted 3,522 18.70 Exercised (609 ) 10.98 Forfeited or expired (775 ) 15.71 Outstanding at December 31, 2020 10,225 $ 14.77 7.8 $ 4,497 Exercisable at December 31, 2020 4,325 $ 13.66 6.6 $ 2,539 During the years ended December 31, 2020, 2019 and 2018, the Company granted stock options to purchase an aggregate of 3,522,258 shares, 4,222,693 shares, and 2,537,277 shares, respectively, at weighted-average grant date fair values per option share of $11.69, $6.99, and $9.49, respectively. The total grant date fair value of options that vested during the years ended December 31, 2020, 2019 and 2018 was $17.4 million, $13.2 million, and $12.1 million, respectively. The aggregate intrinsic value of stock options exercised was $5.5 million in 2020, $1.2 million in 2019 and $1.5 million in 2018. As of December 31, 2020, there was $48.2 million in unrecognized stock-based compensation related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.6 years. Restricted Stock Units During the year-ended December 31, 2020, 541,431 restricted stock units (“RSUs”) were granted to executives and employees. The awards were service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years. Number of Units Weighted Average Grant Date Fair Value per Unit (In thousands except per share data) Outstanding at December 31, 2019 284 $ 9.34 Granted 541 19.64 Vested (71 ) 10.59 Forfeited (86 ) 13.33 Outstanding at December 31, 2020 668 $ 17.56 Compensation expense totaling $2.7 million and $0.5 million was recognized for the service-based RSUs for the years-ended December 31, 2020 and December 31, 2019, respectively. As of December 31, 2020, there was $8.3 million of unrecognized compensation cost related to service-based RSUs that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.7 years. During 2019, the Company granted 604,000 RSUs to executives and employees, which contained performance conditions, 20% of the RSUs vested on June 30, 2019, 25% of the RSUs vested on January 23, 2020, 20% of the RSUs vested on March 24, 2020, and 30% of the RSUs vested on June 25, 2020 in connection with achievement of the final performance milestone. Number of Performance Based RSU Shares Weighted Average Grant Date Fair Value per Unit (In thousands except per share data) Outstanding at December 31, 2019 443 $ 12.16 Granted 17 16.14 Vested (432 ) 12.38 Forfeited (28 ) 11.95 Outstanding at December 31, 2020 — $ — Compensation expense totaling $3.5 million was recognized for the performance-based RSUs for the year-ended December 31, 2020. There was no unrecognized compensation cost as of December 31, 2020, related to performance-based RSUs, as all of the performance conditions have been achieved. As of December 31, 2020, there were approximately 668,000 RSUs outstanding. 401(k) Savings Plan The Company has a defined contribution 401(k) savings plan (the “401(k) Plan”). The 401(k) Plan covers substantially all employees, and allows participants to defer a portion of their annual compensation on a pretax basis. Company contributions to the 401(k) Plan may be made at the discretion of the board of directors. During the year ended December 31, 2014, the Company implemented a matching contribution to the 401(k) Plan, matching 50% of an employee’s contribution up to a maximum of 3% of the participant’s compensation. Company contributions to the 401(k) plan totaled $1.2 million, $0.6 million and $0.5 million in the years ended December 31, 2020, 2019 and 2018, respectively.

Loss per Share

Loss per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Loss per Share	16. Loss per Share As described in Note 2, Summary of Significant Accounting Policies Basic and diluted loss per share allocable to common stockholders are computed as follows: Year Ended December 31, 2020 2019 2018 (In thousands except per share data) Net loss $ (231,694 ) $ (170,295 ) $ (123,630 ) Accretion of Series A Preferred Stock — (2,940 ) — Net loss attributable to common stockholders $ (231,694 ) $ (173,235 ) $ (123,630 ) Weighted average shares outstanding 100,960 89,891 71,864 Basic and diluted loss per share allocable to common stockholders $ (2.29 ) $ (1.93 ) $ (1.72 ) The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Year Ended December 31, 2020 2019 2018 (In thousands) Stock options 10,225 8,087 5,153 Restricted stock units 669 757 — Shares issuable under employee stock purchase plan 98 38 28 Series A Preferred Stock (if converted) 3,378 3,500 — Warrants 2,500 2,500 — 16,870 14,882 5,181 For the year ended December 31, 2019, the above table does not include the up to 6,250,000 shares subject to the Company’s option to sell additional shares to RPI pursuant to the Put Option as the decision to exercise this option was within the Company’s control. On December 30, 2019, the Company exercised its option to sell 2,500,000 shares of Common Stock to RPI for an aggregate of $50.0 million. The sale was effected on February 11, 2020.

Unaudited Quarterly Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Results	17. Unaudited Quarterly Results The results of operations on a quarterly basis for the years ended December 31, 2020 and 2019 are set forth below: Quarter Ended March 31, 2020 June 30, 2020 September 30, 2020 December 31, 2020 (In thousands, except per share data) Revenue: Product revenue, net $ 1,284 $ 2,234 $ 3,445 $ 4,506 Collaboration revenue 70 233 121 3,869 Total revenue 1,354 2,467 3,566 8,375 Operating expenses: Cost of product revenue 614 1,022 1,608 1,823 Research and development 25,163 26,352 25,738 33,680 Selling, general and administrative 26,927 32,661 30,575 35,015 Total operating expenses 52,704 60,035 57,921 70,518 Operating loss (51,350 ) (57,568 ) (54,355 ) (62,143 ) Other (expense) income, net: Interest (expense) income, net: 756 (569 ) (1,364 ) (3,505 ) Other (expense), net (48 ) (15 ) (42 ) 6 Non-cash interest expense related to sale of future royalties (295 ) (301 ) (312 ) (475 ) Other (expense) income, net: 413 (885 ) (1,718 ) (3,974 ) Income tax (provision) — 2 — (116 ) Net loss $ (50,937 ) $ (58,451 ) $ (56,073 ) $ (66,233 ) Reconciliation of net loss to net loss attributable to common stockholders Net loss $ (50,937 ) $ (58,451 ) $ (56,073 ) $ (66,233 ) Accretion of convertible preferred stock — — — — Net loss attributable to common stockholders $ (50,937 ) $ (58,451 ) $ (56,073 ) $ (66,233 ) Loss per share allocable to common stockholders: Basic $ (0.51 ) $ (0.58 ) $ (0.55 ) $ (0.65 ) Diluted $ (0.51 ) $ (0.58 ) $ (0.55 ) $ (0.65 ) Weighted-average common shares outstanding used in net loss per share attributable to common stockholders: Basic 99,616 101,104 101,512 101,596 Diluted 99,616 101,104 101,512 101,596 Quarter Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 (In thousands, except per share data) Revenue: Product revenue, net $ — $ — $ — $ — Collaboration revenue 7,891 5,900 5,715 4,294 Total revenue 7,891 5,900 5,715 4,294 Operating expenses: Cost of product revenue — — — — Research and development 26,896 40,907 26,579 38,257 Selling, general and administrative 11,986 15,698 17,089 23,530 Total operating expenses 38,882 56,605 43,668 61,787 Operating loss (30,991 ) (50,705 ) (37,953 ) (57,493 ) Other (expense) income, net: Interest income, net: 1,658 2,253 1,879 1,320 Other (expense) income, net (6 ) (13 ) (15 ) 21 Non-cash interest expense related to sale of future royalties — — — (192 ) Other income, net: 1,652 2,240 1,864 1,149 Income tax (provision) — — — (58 ) Net loss $ (29,339 ) $ (48,465 ) $ (36,089 ) $ (56,402 ) Reconciliation of net loss to net loss attributable to common stockholders Net loss $ (29,339 ) $ (48,465 ) $ (36,089 ) $ (56,402 ) Accretion of convertible preferred stock (2,940 ) — — — Net loss attributable to common stockholders $ (32,279 ) $ (48,465 ) $ (36,089 ) $ (56,402 ) Loss per share allocable to common stockholders: Basic $ (0.39 ) $ (0.53 ) $ (0.40 ) $ (0.59 ) Diluted $ (0.39 ) $ (0.53 ) $ (0.40 ) $ (0.59 ) Weighted-average common shares outstanding used in net loss per share attributable to common stockholders: Basic 82,424 90,876 91,044 95,074 Diluted 82,424 90,876 91,044 95,074

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Basis Of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The consolidated financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC, and in accordance with U.S. generally accepted accounting principles, or GAAP. Any reference in these notes to applicable guidance is meant to refer to the authoritative accounting principles generally accepted in the United States as found in the ASC and Accounting Standards Update, or ASU, of the Financial Accounting Standards Board, or FASB. The consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of these consolidated financial statements in accordance with accounting principles generally accepted in the United States requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities, as of the date of the consolidated financial statements, and the reported amounts of revenue and expenses during the reporting period. Actual results and outcomes may differ materially from management’s estimates, judgments and assumptions.
Subsequent Events	Subsequent Events The Company considers events or transactions that occur after the balance sheet date but before the consolidated financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company evaluated all events and transactions through the date these financial statements were filed with the Securities and Exchange Commission.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents are comprised of demand deposit accounts, funds in money market accounts, commercial paper and corporate notes.
Marketable securities	Marketable securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. The Company considers all available-for-sale securities, including those with maturity dates beyond 12 months, as available to support current operational liquidity needs and therefore classifies all securities with maturity dates beyond 90 days at the date of purchase as current assets. Available-for-sale securities are maintained by the Company’s investment managers and may consist of commercial paper, high-grade corporate notes, U.S. Treasury securities, and U.S. government agency securities. Available-for-sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive loss as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2020 was $67.7 million, which consisted of 6 commercial paper securities, 7 corporate notes securities and 1 U.S. Treasury security. There were no marketable securities held by the Company for greater than twelve months as of December 31, 2020. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2019 was $116.7 million, which consisted of 3 commercial paper securities and 19 corporate notes securities. The aggregate fair value of securities held by the Company in an unrealized loss position for greater than twelve months as of December 31, 2019 was $4.0 million, which consisted of 1 U.S. government agency security. The Company does not intend to sell and it is unlikely that the Company will be required to sell the above investments before recovery of their amortized cost bases, which may be maturity. The Company determined there was no material change in the credit risk of the above investments, and as a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of December 31, 2020 and 2019. The Company reviews its portfolio of available-for-sale debt securities, using both quantitative and qualitative factors, to determine if declines in fair value below cost have resulted from a credit-related loss or other factors. If the decline in fair value is due to credit-related factors, a loss is recognized in net income, whereas if the decline in fair value is not due to credit-related factors, the loss is recorded in other comprehensive income (loss). The following table summarizes the available for sale securities held at December 31, 2020 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 158,907 $ 14 $ (8 ) $ 158,913 Corporate notes 33,437 3 (7 ) 33,433 U.S. government agency securities and U.S. Treasuries 13,044 1 — 13,045 Total $ 205,388 $ 18 $ (15 ) $ 205,391 The following table summarizes the available for sale securities held at December 31, 2019 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 96,952 $ 27 $ (16 ) $ 96,963 Corporate notes 140,634 49 (41 ) 140,642 U.S. government agency securities and U.S. Treasuries 4,000 — — 4,000 Total $ 241,586 $ 76 $ (57 ) $ 241,605 At December 31, 2020 and 2019, the Company had no securities and 1 security in an unrealized loss position for greater than twelve months, respectively, which the Company concluded was not impaired. Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. All marketable securities held at December 31, 2020 have maturities of less than one year. The majority of marketable securities held at December 31, 2019 have maturities of less than one year, with the exception of one U.S. government agency security. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2020, the balance in the Company’s accumulated other comprehensive loss was composed mainly of activity related to the Company’s available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the year ended December 31, 2020 and December 31, 2019, respectively, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period.
Restricted Cash	Restricted Cash A reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows, is as follows: December 31, 2020 2019 2018 (In thousands) Cash and cash equivalents $ 168,215 $ 139,482 $ 86,671 Restricted cash, as part of other assets 1,509 1,509 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 169,724 $ 140,991 $ 87,133 The $1.5 million in restricted cash is comprised of $0.5 million in a letter of credit as a security deposit for the office and laboratory lease at Technology Square in Cambridge, Massachusetts and $1.0 million in a letter of credit as a security deposit for the Company’s office lease at Hampshire Street in Cambridge, Massachusetts. The Company has recorded cash held to secure these letters of credit as restricted cash in restricted cash and other assets on the consolidated balance sheet. The restricted cash is classified as non-current based on the related lease terms.
Fair Value Measurements	Fair Value Measurements The FASB Codification Topic 820, Fair Value Measurements and Disclosures, Level 1 Quoted prices in active markets for identical assets or liabilities. Level 2 Observable inputs other than Level 1 prices, 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. The Company’s financial instruments as of December 31, 2020 and 2019 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of December 31, 2020 and December 31, 2019, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of December 31, 2020 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 163,264 $ 113,505 $ 49,759 $ — Marketable securities: Commercial paper 158,913 — 158,913 — Corporate notes 33,433 — 33,433 — U.S. government agency securities and treasuries 13,045 — 13,045 — Total $ 368,655 $ 113,505 $ 255,150 $ — Fair Value as of December 31, 2019 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 132,193 $ 124,419 $ 7,774 $ — Marketable securities: Commercial paper 96,963 — 96,963 — Corporate notes 140,642 — 140,642 — U.S. government agency securities and treasuries 4,000 — 4,000 — Total $ 373,798 $ 124,419 $ 249,379 $ — Cash equivalents and marketable securities have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, utilizing third-party pricing services or other market observable data. The Company measures its cash equivalents at fair value on a recurring basis, which approximates the net asset value per share. The Company classifies some of its cash equivalents within Level 1 of the fair value hierarchy because they are valued using observable inputs that reflect quoted prices for identical assets in active markets. The Company measures its marketable securities at fair value on a recurring basis and classifies those instruments and some cash equivalents within Level 2 of the fair value hierarchy. The pricing services used by management utilize industry standard valuation models, including both income and market-based approaches and observable market inputs to determine the fair value of marketable securities and those cash equivalents classified within Level 2 of the fair value hierarchy. As of December 31, 20 20 , the fair value of the long-term debt, payable in installments through November 18, 202 6 , approximated its carrying value due to the proximity of the issuance of the Tranche D Loan date to December 31, 20 20 ( See Note 13, Long-Term Debt ) .
Amortization of Debt Discount and Issuance Costs	Amortization of Debt Discount and Issuance Costs Long-term debt is initially recorded at its allocated proceeds, net of discounts and deferred costs. Debt discount and issuance costs, consisting of legal and other fees directly related to the debt, are offset against initial carrying value of the debt and are amortized to interest expense over the estimated life of the debt based on the effective interest method.
Liability Related to Sale of Future Royalties	Liability Related to Sale of Future Royalties The Company treats the liability related to sale of future royalties as a debt financing, as the Company has significant continuing involvement in the generation of the cash flows, to be amortized to interest expense using the effective interest rate method over the life of the related royalty stream. The liability related to sale of future royalties and the related interest expense are based on the Company’s current estimates of future royalties expected to be paid over the life of the arrangement. The Company will periodically assess the expected royalty payments using a combination of internal projections and forecasts from external sources. To the extent the Company’s future estimates of royalty payments are greater or less than previous estimates or the estimated timing of such payments is materially different than its previous estimates, the Company will prospectively recognize related non-cash interest expense. For further discussion of the sale of future royalties, refer to Note 12, Sale of Future Royalties
Going Concern	Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to its available cash, cash equivalents and marketable securities. The analysis for the year ended December 31, 2020 included consideration of the Company’s current cash needs, including its research and development plans, commercialization activities associated with the ongoing launch of TAZVERIK in the ES and FL indications, and its existing debt service obligations. The Company also evaluated its forecasted product revenues from sales of TAZVERIK. Such estimates of future sales contain significant judgement as TAZVERIK was recently launched and there is little or no history with which to base such estimates. The Company expects its available cash, cash equivalents and marketable securities will be sufficient to fund current planned operations and capital expenditure requirements for at least the next twelve months from the filing date of this Annual Report on Form 10-K with the SEC. As a result, the Company concluded that it did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. The Company’s current operating plan is based on assumptions that may prove to be wrong, and the Company could use its capital resources sooner than it expects.
Accounts Receivable	Accounts Receivable The Company extends credit to customers based on its evaluation of the customer’s financial condition. The Company records receivables for all billings when amounts are due under standard terms. Accounts receivable are stated at amounts due net of applicable prompt pay discounts and other contractual adjustments as well as an allowance for doubtful accounts. The Company assesses the need for an allowance for doubtful accounts by considering a number of factors, including the length of time trade accounts receivable are past due, the customer’s ability to pay its obligation and the condition of the general economy and the industry as a whole. The Company will write off accounts receivable when the Company determines that they are uncollectible. In general, the Company has experienced no significant collection issues with its customers.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer. Accounts receivable represent amounts due from customers and collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection. For a further discussion of concentration of credit risk see Note 3, Product Revenue, Net .
Property and Equipment	Property and Equipment The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception. The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 3 - 6 years Computer and office equipment, and furniture 3 - 10 years Leasehold improvements 3 - 6 years or term of respective lease, if shorter Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews long-lived assets to be held and used, including property and equipment and intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets or asset group may not be recoverable. Evaluation of recoverability is based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. No such impairments were recorded during 2020, 2019 or 2018.
Income Taxes	Income Taxes The Company records deferred income taxes to recognize the effect of temporary differences between tax and financial statement reporting. The Company calculates the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made. The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0 % likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense. Refer to Note 8 , Income Taxes , for additional information regarding the Company’s income taxes.
Research and Development Expenses	Research and Development Expenses Research and development expenses are expensed as incurred. Research and development expenses are comprised of costs incurred in providing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract research and development services, and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. External research and development expenses associated with the Company’s programs include clinical trial site costs, clinical manufacturing costs, costs incurred for consultants and other outside services, such as data management and statistical analysis support, and materials and supplies used in support of the clinical and preclinical programs. Internal costs of the Company’s clinical programs include salaries, stock-based compensation, and the portion of the Company’s facility costs allocated to research and development expense. When vendors billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those vendors, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history for related activities and the expected duration of the vendor service contract, where applicable. The Company generally accrues expenses related to research and development activities based on the services received and efforts expended pursuant to contracts with multiple contract research organizations that conduct and manage clinical trials, as well as other vendors that provide research and development services. Payments for these activities are based on the terms of the individual arrangements and may result in payment terms that differ from the pattern of costs incurred. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of subjects and the completion of clinical trial milestones. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from estimates, the Company would adjust the accrual or prepaid accordingly in future periods.
Stock-Based Compensation	Stock-Based Compensation The Company measures employee and non-employee stock-based compensation based on the grant date fair value of the stock-based compensation award. The Company grants stock options at exercise prices equal to the fair value of the Company’s common stock on the date of grant, based on observable market prices. The Company recognizes employee stock-based compensation expense on a straight-line basis over the requisite service period of the awards. The Company recognizes forfeitures at the time they occur. The actual expense recognized over the vesting period will only represent those options that vest. For awards with performance conditions in which the award does not vest unless the performance condition is met, the Company recognizes expense if, and to the extent that, the Company estimates that achievement of the performance condition is probable. If the Company concludes that vesting is probable, it recognizes expense from the date it reaches this conclusion through the estimated vesting date. For awards with performance conditions that accelerate vesting of the award, the Company estimates the likelihood of satisfaction of the performance conditions, which affects the period over which the expense is recognized, and recognizes the expense using the accelerated attribution model. Refer to Note 15, Employee Benefit Plans
Earnings (Loss) per Share	Earnings (Loss) per Share The Company computes basic earnings (loss) per share by dividing income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s restricted stock and Series A Convertible Preferred Stock par value of $0.0001 per share (the “Series A Preferred Stock”) participate in dividends 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, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted earnings (loss) per share after giving consideration to the dilutive effect of stock options and warrants that are outstanding during the period, except where such non-participating securities would be anti-dilutive. Refer to Note 16, Loss per Share
Segment Information	Segment Information The Company currently operates as one reportable business segment: the discovery, development, and commercialization of novel epigenetic therapies for patients with cancer and other diseases.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Financial Instruments – Credit Losses In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses Effective January 1, 2020, the Company adopted ASC 326 using the required modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the guidance in ASC 326. The FASB subsequently issued amendments to ASU 2016-13, which have the same effective date and transition date of January 1, 2020. The adoption of these standards resulted in an immaterial allowance for credit losses on the Company’s consolidated balance sheet. The adoption of these standards did not have a material effect on the Company’s consolidated statements of operations and comprehensive loss or consolidated statements of cash flows.
Inventory	Inventory The Company outsources the manufacturing of TAZVERIK Inventory is composed of raw materials, intermediate materials, which are classified as work-in-process, and finished goods, which are goods that are available for sale. The Company states inventory at the lower of cost or net realizable value with the cost based on the first-in, first-out method. If the Company identifies excess, obsolete or unsalable items, it writes down its inventory to its net realizable value in the period in which the impairment is identified. These adjustments are recorded based upon various factors related to the product, including the level of product manufactured by the Company, the level of product in the distribution channel, current and projected demand, the expected shelf-life of the product and firm inventory purchase commitments. Shipping and handling costs incurred for inventory purchases are included in inventory costs and costs incurred for product shipments are recorded as incurred in cost of product revenue. Prior to receiving its first approval from the U.S. Food and Drug Administration, or FDA, on January 23, 2020 to sell TAZVERIK, the Company expensed all costs incurred related to the manufacture of TAZVERIK as research and development expense because of the inherent risks associated with the development of a product candidate, the uncertainty about the regulatory approval process and the lack of history for the Company of regulatory approval of drug candidates.
Intangible Assets, Net	Intangible Assets, Net Intangible assets consist of capitalized milestone payments made to third parties under an in-license of patent rights upon receiving regulatory approval of TAZVERIK. The finite lived intangible assets are being amortized on a straight-line basis over the expected time period the Company will benefit from the in-licensed rights, which is generally the patent life. Intangible assets are recorded at cost at the time of their acquisition and are stated in the Company’s consolidated balance sheets net of accumulated amortization and impairments, if applicable. The amortization expense is recognized as cost of product revenue in the Company’s consolidated statement of operations. During the first quarter of 2020 the Company paid a $25.0 million milestone payment under its agreement with Eisai, Co., Ltd., or Eisai, upon regulatory approval of tazemetostat for ES. During the second quarter of 2020 the Company paid a $25.0 million milestone payment under its agreement with Eisai upon regulatory approval of tazemetostat for FL The following table presents intangible assets as of December 31, 2020 (in thousands): December 31, 2020 Estimated useful life (years) In-licensed rights $ 50,000 12.2 Less: accumulated amortization (2,998 ) Total intangible asset, net $ 47,002 The Company recorded approximately $3.0 million in amortization expense related to intangible assets, using the straight-line methodology, during the year ended December 31, 2020. Estimated future amortization expense for intangible assets is approximately $4.2 million per each year thereafter. The Company assesses its intangible assets for impairment if indicators are present or changes in circumstance suggest that impairment may exist. Events that could result in an impairment, or trigger an interim impairment assessment, include the receipt of additional clinical or nonclinical data regarding one of the Company’s drug candidates or a potentially competitive drug candidate, changes in the clinical development program for a drug candidate, or new information regarding potential sales for the drug. If impairment indicators are present or changes in circumstance suggest that impairment may exist, the Company performs a recoverability test by comparing the sum of the estimated undiscounted cash flows of each intangible asset to its carrying value on the consolidated balance sheet. If the undiscounted cash flows used in the recoverability test are less than the carrying value, the Company would determine the fair value of the intangible asset and recognize an impairment loss if the carrying value of the intangible asset exceeds its fair value.
Leases	Leases Adoption of Accounting Standards Codification Topic 842, Leases Effective January 1, 2019, the Company adopted Accounting Standards Codification Topic 842, Leases (ASC 842), which requires lessees to recognize assets and liabilities on the balance sheet for the rights and obligations created by all leases with terms of more than 12 months. The Company elected to employ the transitionary relief offered by the FASB under ASU 2018-11 which allowed the new standard to be implemented without the restatement of comparative periods' financial information. The Company also elected to employ the package of practical expedients offered under ASC 842 and as a result did not assess (1) the presence of a lease in any expired or existing contracts, (2) the lease classification for any existing or expired leases, or (3) the initial direct costs for any existing leases. Additionally, the Company elected to account for the lease components and non-lease components as a single lease component. ASU-2018-11 also provides for recognizing the effects of applying ASC 842 as a cumulative-effect adjustment to retained earnings as of January 1, 2019; however, no such adjustment was recorded as of January 1, 2019. The Company recorded the liability associated with the leases at the present value of the lease payments not yet paid, discounted using the discount rate for the leases established at the adoption date. As the discount rate implicit in the leases was typically not readily determinable, the Company utilized the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. incremental borrowing rate (IBR). The adoption of this standard resulted in the recognition of operating lease liabilities and right-of-use assets of $11.5 million and $10.7 million, respectively, on the Company’s consolidated balance sheet relating to its leases for its corporate headquarters and other office space in Cambridge, Massachusetts and other operating leases. The adoption of the standard did not have a material effect on the Company’s consolidated statements of operation and comprehensive loss or consolidated statements of cash flows. The lease liability will be reduced over the remaining lease term based on cash payments made offset by accretion of monthly interest calculated on the lease liability. The right-of-use (ROU) asset will be amortized over the remaining lease term in an amount equal to the difference between the calculated straight-line expense of the total lease payments less the monthly interest calculated on the remaining lease liability. Subsequent Lease Recognition At inception of a contract, we determine whether an arrangement is or contains a lease. For all leases, we determine the classification as either operating or financing. As of December 31, 2020, the Company does not have any leases that are classified as finance leases. ROU assets represent our right to use an underlying asset for the lease term, and lease liabilities represent our obligation to make lease payments under the lease. Lease recognition occurs at the commencement date, and lease liability amounts are based on the present value of lease payments made during the lease term. Renewals are not assumed in the determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease. Because our leases do not provide information to determine an implicit interest rate, we use our incremental borrowing rate in determining the present value of lease payments. ROU assets also include any lease payments made prior to the commencement date less lease incentives received. Operating lease expense is recognized on a straight-line basis over the lease term. The Company has elected not to apply the recognition requirements to short-term leases with a term of 12 months or less. Instead, the Company recognizes the lease payments in the consolidated statements of operations and comprehensive loss on a straight-line basis over the lease term.
Pending Accounting Pronouncements	Pending Accounting Pronouncements In November 2018, the Financial Accounting Standards Board, or the FASB, issued ASU 2018-18, Collaborative Arrangements, or ASC 808, Revenue from Contracts with Customers In December 2019, the FASB issued ASU 2019-12, Income Taxes The Company is currently evaluating the impact of the adoption of ASU 2019-12 and does not expect adoption to have a material effect on the Company’s consolidated financial statements or disclosures.
Product [Member]
Revenue Recognition	Revenue Recognition – Product Revenue The Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for those goods or services. To determine revenue recognition, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. For a further discussion of accounting for net product revenue see Note 3, Product Revenue, Net
Collaboration Revenue [Member]
Revenue Recognition	Revenue Recognition – Collaboration Revenue Under ASC 606, Revenue from Contracts with Customers The Company has entered into collaboration and license agreements, which are within the scope of ASC 606, to discover, develop, manufacture and commercialize product candidates. The terms of these agreements typically contain multiple promises or obligations, which may include: (i) licenses, or options to obtain licenses, to compounds directed to specific targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed targets. Payments to the Company under these agreements may include non-refundable license fees, customer option exercise fees, payments for research activities, reimbursement of certain costs, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company first evaluates license and/or collaboration arrangements to determine whether the arrangement (or part of the arrangement) represents a collaborative arrangement pursuant to ASC Topic 808, Collaborative Arrangements, , which represent a collaborative relationship and not a customer relationship, Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses – If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promises, whether the value of the license is dependent on the unsatisfied promises, whether there are other vendors that could provide the remaining promises, and whether it is separately identifiable from the remaining promises. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services – The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under above. Reimbursements from the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options – The Company’s arrangements may provide a collaborator with the right to select a target for licensing either at the inception of the arrangement or within an initial pre-defined selection period, which may, in certain cases, include the right of the collaborator to extend the selection period. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires. Milestone Payments – At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Royalties – For arrangements that include sales-based royalties, including milestone payments based on a 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). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 11, Collaborations

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available for sale securities held at December 31, 2020 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 158,907 $ 14 $ (8 ) $ 158,913 Corporate notes 33,437 3 (7 ) 33,433 U.S. government agency securities and U.S. Treasuries 13,044 1 — 13,045 Total $ 205,388 $ 18 $ (15 ) $ 205,391 The following table summarizes the available for sale securities held at December 31, 2019 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 96,952 $ 27 $ (16 ) $ 96,963 Corporate notes 140,634 49 (41 ) 140,642 U.S. government agency securities and U.S. Treasuries 4,000 — — 4,000 Total $ 241,586 $ 76 $ (57 ) $ 241,605
Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	A reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows, is as follows: December 31, 2020 2019 2018 (In thousands) Cash and cash equivalents $ 168,215 $ 139,482 $ 86,671 Restricted cash, as part of other assets 1,509 1,509 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 169,724 $ 140,991 $ 87,133
Summary of Company's Financial Assets Recognized at Fair Value	As of December 31, 2020 and December 31, 2019, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of December 31, 2020 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 163,264 $ 113,505 $ 49,759 $ — Marketable securities: Commercial paper 158,913 — 158,913 — Corporate notes 33,433 — 33,433 — U.S. government agency securities and treasuries 13,045 — 13,045 — Total $ 368,655 $ 113,505 $ 255,150 $ — Fair Value as of December 31, 2019 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 132,193 $ 124,419 $ 7,774 $ — Marketable securities: Commercial paper 96,963 — 96,963 — Corporate notes 140,642 — 140,642 — U.S. government agency securities and treasuries 4,000 — 4,000 — Total $ 373,798 $ 124,419 $ 249,379 $ —
Useful Lives for Property, Plant and Equipment	The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives: Asset Category Useful Lives Laboratory equipment 3 - 6 years Computer and office equipment, and furniture 3 - 10 years Leasehold improvements 3 - 6 years or term of respective lease, if shorter
Summary of Intangible Assets	The following table presents intangible assets as of December 31, 2020 (in thousands): December 31, 2020 Estimated useful life (years) In-licensed rights $ 50,000 12.2 Less: accumulated amortization (2,998 ) Total intangible asset, net $ 47,002

Product Revenue Net (Tables)

Product Revenue Net (Tables)	12 Months Ended
	Dec. 31, 2020
Net Product Revenue [Member]
Concentration Risk [Line Items]
Revenue Of Product Revenue Allowance And Reserve Categories	Revenue earned from each customer as a percentage of net product revenue is as follows:
Accounts Receivable [Member]
Concentration Risk [Line Items]
Schedules of Concentration of Risk Percentage	Year Ended December 31, 2020 2019 Customer 1 45 % — Customer 2 11 % — Customer 3 20 % — Customer 4 24 % — As of December 31, 2020, four individual customers accounted for a percentage of accounts receivable as follows: December 31, 2020 December 31, 2019 Customer 1 21 % — Customer 2 14 % — Customer 3 29 % — Customer 4 36 % —

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net consists of the following: December 31, 2020 2019 (In thousands) Laboratory equipment $ 4,435 $ 4,273 Computer and office equipment, and furniture 4,636 5,113 Leasehold improvements 453 424 Construction in progress 34 560 Property and equipment 9,558 10,370 Less: accumulated depreciation and amortization (7,406 ) (8,151 ) Property and equipment, net $ 2,152 $ 2,219

Inventory (Tables)

Inventory (Tables)	12 Months Ended
	Dec. 31, 2020
Inventory Disclosure [Abstract]
Summary of Inventory	All of the Company’s inventory relates to the manufacturing of TAZVERIK. The following table sets forth the Company’s inventory as of December 31, 2020 and December 31, 2019: December 31, 2020 December 31, 2019 (In thousands) Raw materials $ 1,068 $ — Work in process 8,564 — Finished goods 829 — Total $ 10,461 $ —

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: December 31, 2020 2019 (In thousands) Prepaid clinical and manufacturing costs $ 12,646 $ 7,657 Interest receivable on available for sale securities 369 943 Other prepaid expenses and other receivables 4,906 6,923 Total prepaid expenses and other current assets $ 17,921 $ 15,523

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: December 31, 2020 2019 (In thousands) Employee compensation and benefits $ 11,921 $ 7,844 Research and development expenses 10,664 9,706 Professional services and other 5,970 4,999 Accrued expenses $ 28,572 $ 22,565

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Provision for (Benefit from) Income Taxes	The provision for (benefit from) income taxes for the years ended December 31, 2020, 2019, and 2018 is as follows: 2020 2019 2018 (In thousands) Current $ 22 $ (34 ) $ (127 ) Deferred 92 92 184 Total 114 58 57 Income tax provision $ 114 $ 58 $ 57
Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate	A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 2018 Federal statutory income tax rate 21.0 % 21.0 % 21.0 % State income taxes 6.1 6.0 5.7 Research and development and other tax credits 2.0 1.9 2.4 Permanent items (0.6 ) (0.7 ) (0.7 ) Change in valuation allowance (27.9 ) (27.5 ) (28.3 ) Other (0.6 ) (0.7 ) (0.1 ) Effective income tax rate 0.0 % 0.0 % 0.0 %
Deferred Tax Assets (Liabilities) Included in Other Assets in Consolidated Balance Sheet	The Company’s deferred tax assets (liabilities) included in other assets in the consolidated balance sheets consist of the following: December 31, 2020 2019 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 238,792 $ 180,877 Research and development and other credit carryforwards 34,205 29,774 Capitalized start-up costs 768 901 Deferred revenue — 1,029 Accruals and allowances 2,949 1,922 Eisai license payment 11,935 12,840 Stock compensation 7,338 6,489 Other 452 442 Sale of royalty 3,857 3,458 Lease liability 5,462 5,990 Business interest 1,220 — Gross deferred tax assets 306,978 243,722 Deferred tax asset valuation allowance (302,137 ) (237,858 ) Total deferred tax assets 4,841 5,864 Deferred tax liabilities: Depreciation and other (18 ) (40 ) Right of use asset (4,709 ) (5,732 ) Total deferred tax liabilities (4,727 ) (5,772 ) Net deferred tax asset (liability) $ 114 $ 92
Summary of Unrecognized Tax Benefits	The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2020 2019 (In thousands) Unrecognized tax benefits - as of beginning of year $ 6,328 $ 5,743 Gross increases - current period tax positions 832 585 Unrecognized tax benefits - as of end of year $ 7,160 $ 6,328

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Summary of Lease Costs and Company's Operating Leases	The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the years ended December 31, 2020 and 2019: Twelve months ended December 31, Twelve months ended December 31, 2020 2019 Lease cost Operating lease cost $ 6,155 $ 3,771 Variable lease cost 1,764 1,318 Total lease cost $ 7,919 $ 5,089 Other information Operating cash flows used for operating leases $ 4,374 $ 3,648 Weighted average remaining lease term 5.3 years 5.3 years Weighted average discount rate 9.77 % 9.60 %
Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases	Future minimum lease payments under the Company’s non-cancelable operating leases as of December 31, 2020 and 2019, are as follows: 2020 (In thousands) 2021 $ 6,436 2022 6,256 2023 2,984 2024 3,053 Thereafter 6,966 Total lease payments $ 25,695 Less: imputed interest (5,621 ) Total operating lease liabilities at December 31, 2020 $ 20,074
Contractual Commitments

Sale of Future Royalties (Table

Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2020
Deferred Revenue Disclosure [Abstract]
Schedule of Activity of Royalty Obligation	The following table shows the activity of the Royalty Obligation since the transaction inception through December 31, 2020: Year Ended December 31, 2020 (In thousands ) Proceeds from sale of future royalties $ 12,601 Non-cash interest expense recognized 1,575 Liability related to the sale of future royalties - ending balance $ 14,176

Long Term Debt (Tables)

Long Term Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at December 31, 2020 (in thousands): Year Ended December 31, 2020 2021 $ — 2022 — 2023 — 2024 70,000 2025 75,000 2026 75,000 Total minimum payments 220,000 Less amounts representing interest and discount (4,330 ) Less current portion — Long-term debt, net of current portion $ 215,670

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2020 2019 2018 (In thousands) Research and development $ 9,093 $ 6,295 $ 4,083 General and administrative 18,516 11,721 7,921 Total $ 27,609 $ 18,016 $ 12,004
Assumptions Used in Applying Pricing Model	Weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 0.9 % 2.2 % 2.6 % Expected life of options 5.99 years 6.0 years 6.0 years Expected volatility of underlying stock 70.9 % 72.0 % 71.5 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the year ended December 31, 2020: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2019 8,087 $ 12.86 Granted 3,522 18.70 Exercised (609 ) 10.98 Forfeited or expired (775 ) 15.71 Outstanding at December 31, 2020 10,225 $ 14.77 7.8 $ 4,497 Exercisable at December 31, 2020 4,325 $ 13.66 6.6 $ 2,539
Summary of Service Based Restricted Stock Units	During the year-ended December 31, 2020, 541,431 restricted stock units (“RSUs”) were granted to executives and employees. The awards were service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years. Number of Units Weighted Average Grant Date Fair Value per Unit (In thousands except per share data) Outstanding at December 31, 2019 284 $ 9.34 Granted 541 19.64 Vested (71 ) 10.59 Forfeited (86 ) 13.33 Outstanding at December 31, 2020 668 $ 17.56
Summary of Performance Based Restricted Stock Units	During 2019, the Company granted 604,000 RSUs to executives and employees, which contained performance conditions, 20% of the RSUs vested on June 30, 2019, 25% of the RSUs vested on January 23, 2020, 20% of the RSUs vested on March 24, 2020, and 30% of the RSUs vested on June 25, 2020 in connection with achievement of the final performance milestone. Number of Performance Based RSU Shares Weighted Average Grant Date Fair Value per Unit (In thousands except per share data) Outstanding at December 31, 2019 443 $ 12.16 Granted 17 16.14 Vested (432 ) 12.38 Forfeited (28 ) 11.95 Outstanding at December 31, 2020 — $ —

Loss per Share (Tables)

Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Basic and Diluted Loss per Share	Basic and diluted loss per share allocable to common stockholders are computed as follows: Year Ended December 31, 2020 2019 2018 (In thousands except per share data) Net loss $ (231,694 ) $ (170,295 ) $ (123,630 ) Accretion of Series A Preferred Stock — (2,940 ) — Net loss attributable to common stockholders $ (231,694 ) $ (173,235 ) $ (123,630 ) Weighted average shares outstanding 100,960 89,891 71,864 Basic and diluted loss per share allocable to common stockholders $ (2.29 ) $ (1.93 ) $ (1.72 )
Common Stock Equivalents Excluded from Calculation of Diluted Loss per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Year Ended December 31, 2020 2019 2018 (In thousands) Stock options 10,225 8,087 5,153 Restricted stock units 669 757 — Shares issuable under employee stock purchase plan 98 38 28 Series A Preferred Stock (if converted) 3,378 3,500 — Warrants 2,500 2,500 — 16,870 14,882 5,181

Unaudited Quarterly Results (Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Results of Operations on Quarterly Basis	The results of operations on a quarterly basis for the years ended December 31, 2020 and 2019 are set forth below: Quarter Ended March 31, 2020 June 30, 2020 September 30, 2020 December 31, 2020 (In thousands, except per share data) Revenue: Product revenue, net $ 1,284 $ 2,234 $ 3,445 $ 4,506 Collaboration revenue 70 233 121 3,869 Total revenue 1,354 2,467 3,566 8,375 Operating expenses: Cost of product revenue 614 1,022 1,608 1,823 Research and development 25,163 26,352 25,738 33,680 Selling, general and administrative 26,927 32,661 30,575 35,015 Total operating expenses 52,704 60,035 57,921 70,518 Operating loss (51,350 ) (57,568 ) (54,355 ) (62,143 ) Other (expense) income, net: Interest (expense) income, net: 756 (569 ) (1,364 ) (3,505 ) Other (expense), net (48 ) (15 ) (42 ) 6 Non-cash interest expense related to sale of future royalties (295 ) (301 ) (312 ) (475 ) Other (expense) income, net: 413 (885 ) (1,718 ) (3,974 ) Income tax (provision) — 2 — (116 ) Net loss $ (50,937 ) $ (58,451 ) $ (56,073 ) $ (66,233 ) Reconciliation of net loss to net loss attributable to common stockholders Net loss $ (50,937 ) $ (58,451 ) $ (56,073 ) $ (66,233 ) Accretion of convertible preferred stock — — — — Net loss attributable to common stockholders $ (50,937 ) $ (58,451 ) $ (56,073 ) $ (66,233 ) Loss per share allocable to common stockholders: Basic $ (0.51 ) $ (0.58 ) $ (0.55 ) $ (0.65 ) Diluted $ (0.51 ) $ (0.58 ) $ (0.55 ) $ (0.65 ) Weighted-average common shares outstanding used in net loss per share attributable to common stockholders: Basic 99,616 101,104 101,512 101,596 Diluted 99,616 101,104 101,512 101,596 Quarter Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 (In thousands, except per share data) Revenue: Product revenue, net $ — $ — $ — $ — Collaboration revenue 7,891 5,900 5,715 4,294 Total revenue 7,891 5,900 5,715 4,294 Operating expenses: Cost of product revenue — — — — Research and development 26,896 40,907 26,579 38,257 Selling, general and administrative 11,986 15,698 17,089 23,530 Total operating expenses 38,882 56,605 43,668 61,787 Operating loss (30,991 ) (50,705 ) (37,953 ) (57,493 ) Other (expense) income, net: Interest income, net: 1,658 2,253 1,879 1,320 Other (expense) income, net (6 ) (13 ) (15 ) 21 Non-cash interest expense related to sale of future royalties — — — (192 ) Other income, net: 1,652 2,240 1,864 1,149 Income tax (provision) — — — (58 ) Net loss $ (29,339 ) $ (48,465 ) $ (36,089 ) $ (56,402 ) Reconciliation of net loss to net loss attributable to common stockholders Net loss $ (29,339 ) $ (48,465 ) $ (36,089 ) $ (56,402 ) Accretion of convertible preferred stock (2,940 ) — — — Net loss attributable to common stockholders $ (32,279 ) $ (48,465 ) $ (36,089 ) $ (56,402 ) Loss per share allocable to common stockholders: Basic $ (0.39 ) $ (0.53 ) $ (0.40 ) $ (0.59 ) Diluted $ (0.39 ) $ (0.53 ) $ (0.40 ) $ (0.59 ) Weighted-average common shares outstanding used in net loss per share attributable to common stockholders: Basic 82,424 90,876 91,044 95,074 Diluted 82,424 90,876 91,044 95,074

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basis Of Presentation [Line Items]
Issuance of common stock (net of commissions and offering costs), Value		$ 122,708	$ 81,602
Proceeds from issuance of common stock, net of commissions		122,991	$ 81,938
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	May 2013
Cash, cash equivalents, and marketable securities	$ 373,600
Accumulated deficit	(988,713)	(757,019)
Royalty Pharma [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	1,527,400
Non-equity funding through collaboration agreement	243,800
Issuance of common stock (net of commissions and offering costs), Value		$ 78,705
RPI Finance Trust [Member]
Basis Of Presentation [Line Items]
Proceeds from issuance of common stock, net of commissions	150,000
RPI Finance Trust (RPI) and BioPharma Credit Investments V (Master) LP and BioPharma Credit PLC (the Lenders) [Member]
Basis Of Presentation [Line Items]
Fund received from collaborators	368,100
BioPharma Credit Investments V Master LP And BioPharma Credit PLC [Member]
Basis Of Presentation [Line Items]
Debt financing received through loan agreement	218,100
IPO [Member]
Basis Of Presentation [Line Items]
Issuance of common stock (net of commissions and offering costs), Value	$ 839,500

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)SecuritySegment	Dec. 31, 2019USD ($)Security	Dec. 31, 2018USD ($)	Jan. 01, 2019USD ($)
Accounting Policies [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value			$ 67,700,000	$ 116,700,000
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value				4,000,000
Investments with an other-than-temporary impairment			$ 0	$ 0
Number of securities in unrealized loss position greater than twelve months | Security			0	1
Realized gains (losses) recognized on sale or maturity of marketable equity securities			$ 0
Restricted cash			1,500,000
Security deposit			500,000
Impairment of long-lived assets			$ 0	$ 0	$ 0
Largest amount of tax benefit			50.00%
Reportable business segment | Segment			1
Amortization Of Intangible Assets			$ 3,000,000
Estimated future amortization of intangible assets			4,200,000
Operating lease, liabilities			20,074,000
Operating lease, right-of-use assets			17,305,000	$ 21,206,000
ASU 2016-02 [Member]
Accounting Policies [Line Items]
Operating lease, liabilities						$ 11,500,000
Operating lease, right-of-use assets						$ 10,700,000
Eisai [Member]
Accounting Policies [Line Items]
Milestone payments	$ 25,000,000	$ 25,000,000
Letter of Credit [Member] | Technology Square [Member]
Accounting Policies [Line Items]
Security deposit			500,000
Letter of Credit [Member] | Hampshire Street [Member]
Accounting Policies [Line Items]
Security deposit			$ 1,000,000
U.S. Government Agency [Member]
Accounting Policies [Line Items]
Number of securities in unrealized loss position | Security				1
Commercial Paper [Member]
Accounting Policies [Line Items]
Number of securities in unrealized loss position | Security			6	3
Corporate Notes [Member]
Accounting Policies [Line Items]
Number of securities in unrealized loss position | Security			7	19
U.S. Treasury Securities [Member]
Accounting Policies [Line Items]
Number of securities in unrealized loss position | Security			1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 205,388	$ 241,586
Available-For-Sale Securities Held, Unrealized Gains	18	76
Available-For-Sale Securities Held, Unrealized Losses	(15)	(57)
Available-For-Sale Securities Held, Fair Value	205,391	241,605
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	158,907	96,952
Available-For-Sale Securities Held, Unrealized Gains	14	27
Available-For-Sale Securities Held, Unrealized Losses	(8)	(16)
Available-For-Sale Securities Held, Fair Value	158,913	96,963
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	33,437	140,634
Available-For-Sale Securities Held, Unrealized Gains	3	49
Available-For-Sale Securities Held, Unrealized Losses	(7)	(41)
Available-For-Sale Securities Held, Fair Value	33,433	140,642
U.S. Government Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	13,044	4,000
Available-For-Sale Securities Held, Unrealized Gains	1
Available-For-Sale Securities Held, Fair Value	$ 13,045	$ 4,000

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 168,215	$ 139,482	$ 86,671
Restricted cash, as part of other assets	1,509	1,509	462
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 169,724	$ 140,991	$ 87,133

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 163,264	$ 132,193
Total	368,655	373,798
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	158,913	96,963
Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	33,433	140,642
U.S. Government Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	13,045	4,000
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	113,505	124,419
Total	113,505	124,419
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	49,759	7,774
Total	255,150	249,379
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	158,913	96,963
Level 2 [Member] | Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	33,433	140,642
Level 2 [Member] | U.S. Government Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	$ 13,045	$ 4,000

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	12 Months Ended
	Dec. 31, 2020
Laboratory Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Laboratory Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	6 years
Computer and Office Equipment and Furniture [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Computer and Office Equipment and Furniture [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Summary of Intangible Assets (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
In-licensed rights	$ 50,000
Less: accumulated amortization	(2,998)
Total intangible asset, net	$ 47,002
Estimated useful life (years)	12 years 2 months 12 days

Product Revenue Net - Summary o

Product Revenue Net - Summary of Product Revenue Allowance and Reserve Categories (Detail) - TAZVERIK [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Valuation And Qualifying Accounts Disclosure [Line Items]
Provision	$ 1,915
Payments or credits	(1,287)
Ending Balance	628
Chargebacks, Discounts, and Fees [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Provision	802
Payments or credits	(669)
Ending Balance	133
Government and Other Rebates [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Provision	1,046
Payments or credits	(618)
Ending Balance	428
Returns [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Provision	67
Ending Balance	$ 67

Product Revenue Net - Additiona

Product Revenue Net - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020Customer	Dec. 31, 2019USD ($)Customer
Accounts Receivable [Member] | Credit Concentration Risk [Member]
Product Information [Line Items]
Number of customers | Customer	4	4
TAZVERIK [Member]
Product Information [Line Items]
Total revenue | $		$ 0

Product Revenue Net - Schedule

Product Revenue Net - Schedule Of Each Customer As a Percentage Of Net Product Revenue (Detail) - Net Product Revenue [Member] - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2020
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	45.00%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	11.00%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	24.00%

Product Revenue Net - Schedul_2

Product Revenue Net - Schedule Of Each Customer As a Percentage Of Accounts Receivable (Detail) - Accounts Receivable [Member] - Credit Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2020
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	21.00%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	29.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	36.00%

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment	$ 9,558	$ 10,370
Less: accumulated depreciation and amortization	(7,406)	(8,151)
Property and equipment, net	2,152	2,219
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,435	4,273
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,636	5,113
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	453	424
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 34	$ 560

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 3,984	$ 840	$ 1,052

Inventory - Summary of Inventor

Inventory - Summary of Inventory (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Inventory Disclosure [Abstract]
Raw materials	$ 1,068
Work in process	8,564
Finished goods	829
Total	$ 10,461

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid clinical and manufacturing costs	$ 12,646	$ 7,657
Interest receivable on available for sale securities	369	943
Other prepaid expenses and other receivables	4,906	6,923
Total prepaid expenses and other current assets	$ 17,921	$ 15,523

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 11,921	$ 7,844
Research and development expenses	10,664	9,706
Professional services and other	5,970	4,999
Accrued expenses	$ 28,572	$ 22,565

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for (Benefit from) Income Taxes (Detail) - USD ($)	3 Months Ended			12 Months Ended
	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Current				$ 22,000	$ (34,000)	$ (127,000)
Deferred				92,000	92,000	184,000
Total	$ 116,000	$ (2,000)	$ 58,000	114,000	58,000	57,000
Income tax provision	$ 116,000	$ (2,000)	$ 58,000	$ 114,000	$ 58,000	$ 57,000

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%	21.00%
State income taxes	6.10%	6.00%	5.70%
Research and development and other tax credits	2.00%	1.90%	2.40%
Permanent items	(0.60%)	(0.70%)	(0.70%)
Change in valuation allowance	(27.90%)	(27.50%)	(28.30%)
Other	(0.60%)	(0.70%)	(0.10%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets (Liabilities) Included in Other Assets in Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 238,792	$ 180,877
Research and development and other credit carryforwards	34,205	29,774
Capitalized start-up costs	768	901
Deferred revenue		1,029
Accruals and allowances	2,949	1,922
Eisai license payment	11,935	12,840
Stock compensation	7,338	6,489
Other	452	442
Sale of royalty	3,857	3,458
Lease liability	5,462	5,990
Business interest	1,220
Gross deferred tax assets	306,978	243,722
Deferred tax asset valuation allowance	(302,137)	(237,858)
Total deferred tax assets	4,841	5,864
Deferred tax liabilities:
Depreciation and other	(18)	(40)
Right of use asset	(4,709)	(5,732)
Total deferred tax liabilities	(4,727)	(5,772)
Net deferred tax asset (liability)	$ 114	$ 92

Income Taxes (Tax Cuts and Jobs

Income Taxes (Tax Cuts and Jobs Act) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
Deferred income tax expense (benefit), reversal of valuation allowance			$ (368,000)
Income tax receivable		$ 184,000
Deferred income taxes	$ 92,000	$ 92,000	$ 184,000
Income tax benefit	0
CARES Act [Member]
Income Tax Contingency [Line Items]
Percentage of refundable tax credit		100.00%
Deferred tax asset related to AMT credit	$ 0

Income Taxes (Operating Loss Ca

Income Taxes (Operating Loss Carryforwards) - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
United States, Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 877.8
Operating loss carryforwards, expire at various dates from 2029 through 2037	$ 428.5
Operating loss carryforwards, expiration beginning year	2029
Operating loss carryforwards, expire indefinitely	$ 449.2
Operating loss carryforwards, expiration ending year	2030
United States, State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 908
Operating loss carryforwards, expiration beginning year	2037

Income Taxes (Research and Deve

Income Taxes (Research and Development Tax Credit) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Tax Credit Carryforward [Line Items]
Research and development tax credit carryforward expiration beginning year	2028
Research and development tax credit carryforward expiration ending year	2040
Federal orphan drug tax credit carryforwards expiration beginning year	2033
Expiry period of federal orphan drug tax credit carryforwards	2040
Federal orphan drug tax credit carryforwards	$ 17,700,000
Accrued tax-related interest or penalties	0	$ 0
Research Tax Credit Carryforward [Member] | United States, Federal [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	12,700,000
Research Tax Credit Carryforward [Member] | United States, State [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	$ 4,800,000

Income Taxes - Summary of Unrec

Income Taxes - Summary of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Unrecognized tax benefits - as of beginning of year	$ 6,328	$ 5,743
Gross increases - current period tax positions	832	585
Unrecognized tax benefits - as of end of year	$ 7,160	$ 6,328

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 30, 2019	Jul. 31, 2019	Mar. 31, 2018
Other Commitments [Line Items]
Regulatory milestone payments obligation	$ 0.9
Reimbursements receivable of regulatory milestone	1
Roche [Member]
Other Commitments [Line Items]
Regulatory milestone payments obligation	1				$ 1
Payments pursuant to companion diagnostic agreement	9.4
Development costs	3.4	$ 4	$ 2
Royalty Pharma [Member] | Roche [Member]
Other Commitments [Line Items]
Remaining development costs	1					$ 10.4
Reimbursements receivable of development costs	0.9
Signed Amendment | Roche [Member]
Other Commitments [Line Items]
Regulatory milestone payments obligation					0.5
Fourth Amendment | Roche [Member]
Other Commitments [Line Items]
Regulatory milestone payments obligation				$ 0.5	$ 0.5
Gsk
Other Commitments [Line Items]
Reimbursements receivable of regulatory milestone	$ 0.9

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2018
Lessee Lease Description [Line Items]
Operating lease, existence of option to terminate	true
Monthly base rent	$ 200,000
Increase In base rent	33,000
Annual increase of base rent	9,000
Security deposit	500,000
Rent expense		$ 3,500,000
Hampshire Street [Member] | Letter of Credit [Member]
Lessee Lease Description [Line Items]
Security deposit	$ 1,000,000
BMR-Hampshire LLC [Member] | Cambridge [Member]
Lessee Lease Description [Line Items]
Operating lease description	the Company entered into a lease, or the Hampshire Street Lease, with BMR-Hampshire LLC, or BMR. The Hampshire Street Lease is for 33,525 rentable square feet of office space in Cambridge, Massachusetts. The Hampshire Street Lease commenced as of December 1, 2019
Operating lease initial term from commencement date	7 years 4 months
Operating lease extend description	the Company with an option to extend the lease term for one additional five-year period
Operating lease rent expense	$ 200,000
Operating lease rent expense incremental percentage	2.50%

Leases - Summary of Lease Costs

Leases - Summary of Lease Costs and Company's Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 6,155	$ 3,771
Variable lease cost	1,764	1,318
Total lease cost	7,919	5,089
Operating cash flows used for operating leases	$ 4,374	$ 3,648
Weighted average remaining lease term	5 years 3 months 18 days	5 years 3 months 18 days
Weighted average discount rate	9.77%	9.60%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
2021	$ 6,436
2022	6,256
2023	2,984
2024	3,053
Thereafter	6,966
Total lease payments	25,695
Less: imputed interest	(5,621)
Total operating lease liabilities at December 31, 2020	$ 20,074

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jan. 01, 2011OptionTarget	Nov. 30, 2018USD ($)Installment	Mar. 31, 2015USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Jan. 31, 2020USD ($)	Dec. 30, 2019USD ($)	Jul. 31, 2019USD ($)	Mar. 31, 2018USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation					$ 900,000
Reimbursements receivable of regulatory milestone					1,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation					$ 1,000,000									$ 1,000,000
Notice period in days					90 days
Gsk
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTarget	3
Clinical development milestone payment					$ 50,000,000
Additional milestone payments					197,000,000
Sales-based milestone payments					128,000,000
Additional payments received					0
Cash and accounts receivable									$ 89,000,000
Total revenue							$ 20,000,000
Deferred revenue					0	$ 0			0
Reimbursements receivable of regulatory milestone					900,000
Gsk | Collaboration Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment received								$ 89,000,000
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment made			$ 40,000,000	$ 10,000,000
Clinical development milestone payments obligation				10,000,000	20,000,000
Regulatory milestone payments obligation					50,000,000
Research and development reduction					5,300,000	3,800,000
Obligation related to World Wide Royalty					1,700,000
Royalties payable					700,000
Other Receivables					100,000	1,300,000			1,300,000
Eisai [Member] | Second Submission of NDA or MAA [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payments obligation				$ 10,000,000		10,000,000			10,000,000
Regulatory milestone payments obligation					25,000,000							$ 25,000,000
Royalty Pharma [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments					1,000,000										$ 10,400,000
Reimbursements receivable of development costs					900,000
Signed Amendment | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation														500,000
Fourth Amendment | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation													$ 500,000	$ 500,000
Boehringer Ingelheim [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment received						15,000,000
Total revenue		$ 26,000,000			500,000	23,800,000
Deferred revenue					0	0			0
Other Receivables					0	1,300,000			1,300,000
Company received upfront payment		15,000,000
Research funding for costs to be incurred		$ 5,000,000			500,000	$ 5,000,000			$ 5,000,000	$ 100,000	$ 400,000		$ 23,800,000
Research funding costs, payment frequency		quarterly
Research funding costs, payable installments | Installment		4
Maximum extension term of research period		1 year
Total transaction price					20,000,000
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Total revenue					99,200,000
Deferred revenue					$ 3,800,000

Sale of Future Royalties - Addi

Sale of Future Royalties - Additional Information (Details)	Dec. 30, 2019USD ($)shares	Nov. 04, 2019USD ($)Tranche$ / sharesshares	Feb. 29, 2020USD ($)shares	Nov. 30, 2019$ / sharesshares	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Nov. 03, 2020USD ($)	Mar. 31, 2019shares	Oct. 31, 2018shares
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares	97,783,000				101,627,000				11,500,000	9,583,334
Proceeds from issuance of common stock, net of commissions						$ 122,991,000	$ 81,938,000
Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Proceeds from sale of future royalties					$ 12,601,000
RPI and Affiliates [Member]
Line Of Credit Facility [Line Items]
Percentage of ownership interest related parties					9.00%	6.80%
Tazemetostat [Member] | Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Non cash royalty revenue					$ 0
Non cash interest expense					200,000
RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares	2,500,000	6,666,667		6,666,667
Warrants to purchase shares of common stock exercise price | $ / shares		$ 20		$ 20
Proceed from agreement		$ 100,000,000
Obligation to purchase common stock		50,000,000
Shares option to sell exercised by underwriters | shares			2,500,000
Proceeds from issuance of common stock, net of commissions	$ 50,000,000		$ 50,000,000
Proceeds from sale of future royalties					$ 12,600,000
Effective annual interest Rate					9.01%
RPI Purchase Agreement [Member] | RPI and Affiliates [Member]
Line Of Credit Facility [Line Items]
Payments for Royalties					13.40%
RPI Purchase Agreement [Member] | Achievement of Specified Annual Net Sales [Member]
Line Of Credit Facility [Line Items]
Potential milestone payments		$ 1,500,000,000
Maximum [Member] | RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Warrants to purchase shares of common stock | shares		2,500,000		2,500,000
Obligation to purchase share price | $ / shares		$ 20
Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 70,000,000
Number of tranches | Tranche		3
Debt instrument, covenant description					Under the Amended and Restated Loan Agreement the Company has the right to request up to an additional $150.0 million in secured term loans, subject to the approval of the Lenders, provided that the Company has not prepaid any outstanding term loans at the time of the Company’s request and such request is made before November 18, 2021.
Loan Agreement [Member] | RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares					79,000,000
Warrants to purchase shares of common stock | shares					8,400,000
Proceed from agreement					$ 125,000,000
Proceeds from sale of future royalties					12,600,000
Transaction costs					2,000,000
Loan Agreement [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Right to request for additional loan amount					150,000,000
First Tranche [Member] | Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 25,000,000
Aggregate principal amount					25,000,000
First Tranche [Member] | Loan Agreement [Member] | RPI Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Aggregate principal amount					25,000,000
Second Tranche [Member]
Line Of Credit Facility [Line Items]
Aggregate principal amount					$ 25,000,000
Second Tranche [Member] | Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		25,000,000
Third Tranche [Member] | Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 20,000,000
Fourth Tranche [Member] | Amended And Restated Loan Agreement [Member]
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity								$ 150,000,000

Sale of Future Royalties - Sche

Sale of Future Royalties - Schedule of Activity of Royalty Obligation (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Deferred Revenue Arrangement [Line Items]
Non-cash interest expense recognized	$ 475	$ 312	$ 301	$ 295	$ 192	$ 1,383	$ 192
Related party liability related to sale of future royalties	14,176				$ 12,793	14,176	$ 12,793
Royalty Purchase Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Proceeds from sale of future royalties						12,601
Non-cash interest expense recognized						1,575
Related party liability related to sale of future royalties	$ 14,176					$ 14,176

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail)	Nov. 03, 2020USD ($)Installment	Nov. 30, 2020USD ($)	Nov. 30, 2019USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Nov. 04, 2019USD ($)
Tranche Term Loan [Member]
Debt Instrument [Line Items]
Interest rate of loan	7.75%
Number of quarterly payments of term loan | Installment	8
Description of prepayment of tranche term loan				Each of the four term loans may be prepaid before maturity in whole or in part, however there is a $50.0 million minimum prepayment for any prepayment of the loans. If the Company prepays any tranche of term loans, in whole or in part, during the first 36 months from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to the greater of (x) a make-whole amount equal to the interest that would have accrued on the principal amount to be prepaid and (y) a premium equal to 0.03 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loan, in whole or in part, between the 36th month and 48th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.02 multiplied by the principal amount to be prepaid. If the Company prepays a tranche of term loans, in whole or in part, between the 48th month and 60th month from the date on which the Lenders funded such tranche of term loans, then the Company must pay a prepayment premium equal to 0.01 multiplied by the principal amount to be prepaid.
Tranche Term Loan [Member] | Interest Rate Floor [Member]
Debt Instrument [Line Items]
Interest rate of loan	2.00%
First Tranche [Member] | Loan Agreement [Member] | BioPharma Credit Investments V (Master) LP [Member]
Debt Instrument [Line Items]
Interest Expense Credit Facilities				$ 7,200,000	$ 300,000
Minimum [Member] | Tranche Term Loan [Member]
Debt Instrument [Line Items]
Prepayment of loan	$ 50,000,000
Loan Agreement [Member]
Debt Instrument [Line Items]
Aggregate principal amount						$ 70,000,000
Percentage of commitment fee paid		2.00%	2.00%
Commitment fee		$ 150,000,000	$ 70,000,000
Loan Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Right to request for additional loan amount						$ 300,000,000
Amended And Restated Loan Agreement [Member]
Debt Instrument [Line Items]
Aggregate principal amount	$ 150,000,000
Number of quarterly payments of term loan | Installment	4
Debt intrument outstanding interest only payment	$ 70,000,000
Principal And Interest Due	$ 70,000,000
Amended And Restated Loan Agreement [Member] | Tranche Term Loan [Member]
Debt Instrument [Line Items]
Number of quarterly payments of term loan | Installment	8
Amended And Restated Loan Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Right to request for additional loan amount	$ 150,000,000

Long-Term Debt - Schedule of Mi

Long-Term Debt - Schedule of Minimum Aggregate Future Loan Payments (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Long-term Debt, Fiscal Year Maturity [Abstract]
2021	$ 0
2022	0
2023	0
2024	70,000
2025	75,000
2026	75,000
Total minimum payments	220,000
Less amounts representing interest and discount	(4,330)
Less current portion	0
Long-term debt, net of current portion	$ 215,670	$ 23,309

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Nov. 04, 2019	Mar. 06, 2019	Feb. 29, 2020	Nov. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 30, 2019	Mar. 31, 2019	Oct. 31, 2018
Equity [Line Items]
Number of votes for each share of common stock					Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders.
Common stock, shares authorized					150,000,000			125,000,000
Common stock, par value					$ 0.0001			$ 0.0001
Common stock, shares issued					101,627,000			97,783,000	11,500,000	9,583,334
Issuance of common stock (net of commissions and offering costs), Value						$ 122,708	$ 81,602
Common stock shares reserved for issuance					20,405,623
Preferred stock, shares issued					338,000			350,000
Net proceeds from sale of shares						$ 37,432
Preferred stock, par value					$ 0.0001			$ 0.0001
Series A Convertible Preferred Stock [Member]
Equity [Line Items]
Preferred stock, shares issued		350,000
Purchase price per share		$ 115
Net proceeds from sale of shares		$ 37,400
Preferred stock, par value		$ 0.0001
Common stock price per share					$ 12.34
Intrinsic value of beneficial conversion feature, discount on share					$ 2,900
Convertible preferred Stock, Conversion Price Per Share					$ 11.50
Preferred stock, voting rights					Shares of Series A Preferred Stock will generally have no voting rights except as required by law and except that the consent of the holders of a majority of our outstanding shares of Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock or take certain other actions with respect to the Series A Preferred Stock.
Liquidation preference per share					$ 0.001
Percentage of conversion restriction upon holder of convertible preferred stock shares					9.99%
Conversion of stock notice period					61 days
Number of common shares convertible for each share of convertible preferred stock					10
Preferred Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares			12,200
Common Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares			122,000
RPI Purchase Agreement [Member]
Equity [Line Items]
Common stock, shares issued	6,666,667			6,666,667				2,500,000
Warrants to exercise common stock	$ 20			$ 20
RPI Purchase Agreement [Member] | Maximum [Member]
Equity [Line Items]
Warrants to purchase shares of common stock	2,500,000			2,500,000
Common Stock [Member]
Equity [Line Items]
Common stock, shares authorized					150,000,000	125,000,000
Common stock, par value					$ 0.0001
Issuance of common stock (net of commissions and offering costs), Value			$ 49,900			$ 1	$ 1
Issuance of common stock upon exercise of stock options, Share			2,500,000			11,500,000	9,583,334
Proceeds from financing Costs after deducting			$ 100
Common Stock [Member] | Series A Convertible Preferred Stock [Member]
Equity [Line Items]
Issuance of Series A Convertible Preferred Stock, net of commissions and beneficial conversion charge, Shares					122,000
Additional Paid-In Capital [Member]
Equity [Line Items]
Issuance of common stock (net of commissions and offering costs), Value						$ 122,707	$ 81,601
Additional Paid-In Capital [Member] | RPI Purchase Agreement [Member]
Equity [Line Items]
Proceeds from sale of equity securities fair value				$ 8,400

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options, expiration period	10 years
Stock-based compensation expense related to stock options, restricted stock, shares issued and shares granted to non-employee directors	$ 27,609	$ 18,016	$ 12,004
Non-employee stock option awards granted	0	0	0
Expected dividend yield	0.00%
Aggregate stock options granted to employees, including a non-employee	3,522,258	4,222,693	2,537,277
Weighted-average fair value of options granted	$ 11.69	$ 6.99	$ 9.49
Grant date fair value of options vested	$ 17,400	$ 13,200	$ 12,100
Aggregate intrinsic value of stock option exercised	5,500	1,200	1,500
Unrecognized compensation cost	$ 48,200
Expected weighted average period for recognition of compensation cost	2 years 7 months 6 days
Percentage of employer's matching contribution				50.00%
Maximum percentage of employee contribution				3.00%
Contribution to 401 (k) plan	$ 1,200	600	$ 500
Defined contribution plan name	401(k) savings plan
Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Vesting terms	25.0% vesting upon the one-year anniversary of the grant and the remaining 75.0% vesting monthly over the following three years
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Restricted stock units outstanding	668,000
Non Qualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Vesting terms	25% vesting upon the one-year anniversary of grant and the remaining 75% vesting monthly over the following three years
Service Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Vesting percentage	25.00%
Expected weighted average period for recognition of compensation cost	2 years 8 months 12 days
Restricted stock units outstanding	541,431
Compensation expense was recognized	$ 2,700	$ 500
Unrecognized compensation cost	$ 8,300
Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	17,000
Compensation expense was recognized	$ 3,500
Unrecognized compensation cost	$ 0
Vesting Year One [Member] | Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%
Vesting Year One [Member] | Non Qualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%
Vesting Remaining Years [Member] | Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	75.00%
Vesting Remaining Years [Member] | Non Qualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	75.00%
Vest on June 30, 2019 [Member] | Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	20.00%
Vest on December 31, 2019 [Member] | Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%
Vest on March 31, 2020 [Member] | Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	20.00%
Vest on September 30, 2020 [Member] | Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	30.00%

Employee Benefit Plans - Schedu

Employee Benefit Plans - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 27,609	$ 18,016	$ 12,004
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	9,093	6,295	4,083
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 18,516	$ 11,721	$ 7,921

Employee Benefit Plans - Assump

Employee Benefit Plans - Assumptions Used in Applying Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.90%	2.20%	2.60%
Expected life of options	5 years 11 months 26 days	6 years	6 years
Expected volatility of underlying stock	70.90%	72.00%	71.50%
Expected dividend yield	0.00%	0.00%	0.00%

Employee Benefit Plans - Summar

Employee Benefit Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	8,087,000
Number of Options, Granted	3,522,258	4,222,693	2,537,277
Number of Options, Exercised	(609,000)
Number of Options, Forfeited or expired	(775,000)
Number of Options, Outstanding, Ending balance	10,225,000	8,087,000
Number of Options, Exercisable	4,325,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 12.86
Weighted Average Exercise Price per Share, Granted	18.70
Weighted Average Exercise Price per Share, Exercised	10.98
Weighted Average Exercise Price per Share, Forfeited or expired	15.71
Weighted Average Exercise Price per Share, Outstanding, Ending balance	14.77	$ 12.86
Weighted Average Exercise Price per Share, Exercisable	$ 13.66
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 9 months 18 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding	$ 4,497
Aggregate Intrinsic Value, Exercisable	$ 2,539

Employee Benefit Plans - Summ_2

Employee Benefit Plans - Summary of Service Based Restricted Stock Units (Detail) - Service Based RSU [Member]	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Outstanding Shares, Beginning Balance | shares	284,000
Number of Shares, Granted | shares	541,431
Number of Shares, Vested | shares	(71,000)
Number of Shares, Forfeited | shares	(86,000)
Number of Outstanding Shares, Ending Balance | shares	668,000
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 9.34
Weighted Average Grant Date Fair Value, Granted | $ / shares	19.64
Weighted Average Grant Date Fair Value, Vested | $ / shares	10.59
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	13.33
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 17.56

Employee Benefit Plans - Summ_3

Employee Benefit Plans - Summary of Performance Based Restricted Stock Units (Detail) - Performance Based RSU [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Outstanding Shares, Beginning Balance | shares	443
Number of Shares, Granted | shares	17
Number of Shares, Vested | shares	(432)
Number of Shares, Forfeited | shares	(28)
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 12.16
Weighted Average Grant Date Fair Value, Granted | $ / shares	16.14
Weighted Average Grant Date Fair Value, Vested | $ / shares	12.38
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	$ 11.95

Loss per Share - Additional Inf

Loss per Share - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 30, 2019	Feb. 29, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Nov. 30, 2019	Nov. 04, 2019	Oct. 31, 2018
Accretion of series A convertible preferred stock			$ 2,940	$ 2,940
Common stock, shares issued	97,783,000		11,500,000			101,627,000			9,583,334
Proceeds from issuance of common stock, net of commissions				$ 122,991	$ 81,938
RPI Purchase Agreement [Member]
Common stock, shares issued	2,500,000						6,666,667	6,666,667
Proceeds from issuance of common stock, net of commissions	$ 50,000	$ 50,000
RPI Purchase Agreement [Member] | Maximum [Member]
Additional shares excluded from the additional option to sell						6,250,000

Loss per Share - Schedule of Ba

Loss per Share - Schedule of Basic and Diluted Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss	$ (66,233)	$ (56,073)	$ (58,451)	$ (50,937)	$ (56,402)	$ (36,089)	$ (48,465)	$ (29,339)	$ (231,694)	$ (170,295)	$ (123,630)
Accretion of convertible preferred stock								(2,940)		(2,940)
Net loss attributable to common stockholders	$ (66,233)	$ (56,073)	$ (58,451)	$ (50,937)	$ (56,402)	$ (36,089)	$ (48,465)	$ (32,279)	$ (231,694)	$ (173,235)	$ (123,630)
Weighted average shares outstanding									100,960	89,891	71,864
Basic and diluted loss per share allocable to common stockholders									$ (2.29)	$ (1.93)	$ (1.72)

Loss per Share - Common Stock E

Loss per Share - Common Stock Equivalents from Calculation of Diluted Loss per Share Attributable to Common Stockholders (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	16,870	14,882	5,181
Employee Stock Option [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	10,225	8,087	5,153
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	669	757
Shares Issuable Under Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	98	38	28
Series A Preferred Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,378	3,500
Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	2,500	2,500

Unaudited Quarterly Results - S

Unaudited Quarterly Results - Summary of Results of Operations on a Quarterly Basis (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Total revenue	$ 8,375	$ 3,566	$ 2,467	$ 1,354	$ 4,294	$ 5,715	$ 5,900	$ 7,891	$ 15,762	$ 23,800	$ 21,700
Operating expenses:
Cost of product revenue	1,823	1,608	1,022	614					5,067
Research and development	33,680	25,738	26,352	25,163	38,257	26,579	40,907	26,896	110,933	132,639	105,833
Selling, general and administrative	35,015	30,575	32,661	26,927	23,530	17,089	15,698	11,986	125,178	68,303	43,972
Total operating expenses	70,518	57,921	60,035	52,704	61,787	43,668	56,605	38,882	241,178	200,942	149,805
Operating loss	(62,143)	(54,355)	(57,568)	(51,350)	(57,493)	(37,953)	(50,705)	(30,991)	(225,416)	(177,142)	(128,105)
Other (expense) income, net:
Interest (expense) income, net:	(3,505)	(1,364)	(569)	756	1,320	1,879	2,253	1,658
Other (expense) income, net	6	(42)	(15)	(48)	21	(15)	(13)	(6)	(99)	(13)	(25)
Related party non-cash interest expense related to sale of future royalties	(475)	(312)	(301)	(295)	(192)				(1,383)	(192)
Other (expense) income, net:	(3,974)	(1,718)	(885)	413	1,149	1,864	2,240	1,652
Income tax provision	(116)		2		(58)				(114)	(58)	(57)
Net loss	(66,233)	(56,073)	(58,451)	(50,937)	(56,402)	(36,089)	(48,465)	(29,339)	(231,694)	(170,295)	(123,630)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(66,233)	(56,073)	(58,451)	(50,937)	(56,402)	(36,089)	(48,465)	(29,339)	(231,694)	(170,295)	(123,630)
Accretion of convertible preferred stock								(2,940)		(2,940)
Net loss attributable to common stockholders	$ (66,233)	$ (56,073)	$ (58,451)	$ (50,937)	$ (56,402)	$ (36,089)	$ (48,465)	$ (32,279)	$ (231,694)	$ (173,235)	$ (123,630)
Loss per share allocable to common stockholders:
Basic	$ (0.65)	$ (0.55)	$ (0.58)	$ (0.51)	$ (0.59)	$ (0.40)	$ (0.53)	$ (0.39)	$ (2.29)	$ (1.93)	$ (1.72)
Diluted	$ (0.65)	$ (0.55)	$ (0.58)	$ (0.51)	$ (0.59)	$ (0.40)	$ (0.53)	$ (0.39)	$ (2.29)	$ (1.93)	$ (1.72)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders:
Basic	101,596	101,512	101,104	99,616	95,074	91,044	90,876	82,424	100,960	89,891	71,864
Diluted	101,596	101,512	101,104	99,616	95,074	91,044	90,876	82,424	100,960	89,891	71,864
Product [Member]
Revenue:
Total revenue	$ 4,506	$ 3,445	$ 2,234	$ 1,284
Collaboration Revenue [Member]
Revenue:
Total revenue	$ 3,869	$ 121	$ 233	$ 70	$ 4,294	$ 5,715	$ 5,900	$ 7,891	$ 4,293	$ 23,800	$ 21,700