Epizyme (EPZM) 10-K 27 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 19, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
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
Entity Common Stock Shares Outstanding		100,771,384
Entity Public Float			$ 992.1
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 2020 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, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 139,482	$ 86,671
Marketable securities	241,605	153,633
Accounts receivable	2,567	20,067
Prepaid expenses and other current assets	15,523	12,164
Total current assets	399,177	272,535
Property and equipment, net	2,219	2,057
Operating lease assets	21,206
Restricted cash and other assets	1,987	909
Total assets	424,589	275,501
Current liabilities:
Accounts payable	8,782	4,780
Accrued expenses	22,549	19,700
Current portion of operating lease obligation	3,039
Current portion of deferred revenue		13,300
Other current liabilities	16	53
Total current liabilities	34,386	37,833
Operating lease obligation, net of current portion	19,120
Deferred revenue, net of current portion	3,806	3,806
Long-term debt, net of debt discount	23,309
Other long-term liabilities	38	853
Liability related to sale of future royalties	12,793
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; 350 shares and no shares issued and outstanding, respectively (equivalent to 3,500 shares of common stock upon conversion at a 10:1 ratio)	37,432
Common stock, $0.0001 par value; 125,000 shares authorized; 97,783 shares and 79,175 shares issued and outstanding, respectively	10	8
Additional paid-in capital	1,050,695	819,779
Accumulated other comprehensive loss	19	(54)
Accumulated deficit	(757,019)	(586,724)
Total stockholders’ equity	331,137	233,009
Total liabilities and stockholders’ equity	$ 424,589	$ 275,501

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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	350,000	0
Preferred stock, shares outstanding	350,000	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	97,783,000	79,175,000
Common stock, shares outstanding	97,783,000	79,175,000
Common stock conversion ratio	10.00%
Common stock upon conversion	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 23,800	$ 21,700	$ 10,000
Type of Revenue [Extensible List]	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember
Operating expenses:
Research and development	$ 132,639	$ 105,833	$ 109,661
General and administrative	68,303	43,972	37,181
Total operating expenses	200,942	149,805	146,842
Operating loss	(177,142)	(128,105)	(136,842)
Other income, net:
Interest income, net	7,110	4,557	2,165
Other (expense) income, net	(13)	(25)	32
Non-cash interest expense related to sale of future royalties	(192)
Other income, net	6,905	4,532	2,197
Loss before income taxes	(170,237)	(123,573)	(134,645)
Income tax (provision) benefit	(58)	(57)	336
Net loss	(170,295)	(123,630)	(134,309)
Other comprehensive loss:
Unrealized gain (loss) on available for sale securities	73	(5)	57
Comprehensive loss	(170,222)	(123,635)	(134,252)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(170,295)	(123,630)	(134,309)
Accretion of convertible preferred stock	(2,940)
Net loss attributable to common stockholders	$ (173,235)	$ (123,630)	$ (134,309)
Net loss per share attributable to common stockholders:
Basic	$ (1.93)	$ (1.72)	$ (2.18)
Diluted	$ (1.93)	$ (1.72)	$ (2.18)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders:
Basic	89,891	71,864	61,471
Diluted	89,891	71,864	61,471

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss		$ (170,295,000)	$ (123,630,000)		$ (134,309,000)	[1]
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		840,000	1,052,000		1,639,000	[1]
Stock-based compensation		18,016,000	12,004,000		11,431,000	[1]
Amortization of discount on investments		(3,175,000)	(1,556,000)		265,000	[1]
Amortization of debt discount		37,000
Non-cash interest expense associated with the sale of future royalties		192,000
Deferred income taxes		92,000	184,000		(368,000)	[1]
Changes in operating assets and liabilities:
Accounts receivable		17,500,000	(19,686,000)		(359,000)	[1]
Prepaid expenses and other current assets		(3,359,000)	(3,181,000)		(2,525,000)	[1]
Accounts payable		3,389,000	(2,404,000)		1,967,000	[1]
Accrued expenses		2,897,000	2,066,000		1,523,000	[1]
Deferred revenue		(13,300,000)	13,300,000
Operating lease assets		(9,921,000)
Operating lease liabilities		10,043,000
Other assets and liabilities		(124,000)	251,000		303,000	[1]
Net cash used in operating activities		(147,168,000)	(121,600,000)		(120,433,000)	[1]
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities		(504,981,000)	(298,670,000)		(126,356,000)	[1]
Proceeds from sales/maturities of marketable securities		420,255,000	196,363,000		240,670,000	[1]
Purchases of property and equipment		(594,000)	(299,000)		(984,000)	[1]
Net cash (used in) provided by investing activities		(85,320,000)	(102,606,000)		113,330,000	[1]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of commissions		122,991,000	81,938,000		152,922,000	[1]
Proceeds from issuance of preferred stock, net of commissions		37,432,000
Payment of public offering costs		(284,000)	(260,000)		(388,000)	[1]
Proceeds from issuance of debt		25,000,000
Payment of debt issuance costs		(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
Payment under capital lease obligation		(16,000)	(129,000)		(620,000)	[1]
Proceeds from stock options exercised		2,358,000	1,885,000		3,281,000	[1]
Issuance of shares under employee stock purchase plan		741,000	779,000		677,000	[1]
Net cash provided by financing activities		286,346,000	84,213,000		155,872,000	[1]
Net (decrease) increase in cash and cash equivalents		53,858,000	(139,993,000)		148,769,000	[1]
Cash, cash equivalents, and restricted cash, beginning of period		87,133,000	227,126,000	[1]	78,357,000	[1]
Cash, cash equivalents, and restricted cash, end of period		140,991,000	87,133,000		227,126,000	[1]
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Unpaid offering costs		78,000	75,000
Unpaid debt issuance costs		78,000
Property and equipment included in accounts payable or accruals		454,000	194,000		58,000	[1]
Cash paid for income taxes		$ 45,000	$ 48,000		33,000	[1]
ASU 2016-09 [Member]
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cumulative adjustment related to the adoption	[1]				$ 115,000
[1]	Revised as a result of the adoption of ASU 2016-18

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Royalty Pharma [Member]	Series A Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Royalty Pharma [Member]	Preferred Stock [Member]	Preferred Stock [Member]Series A Convertible Preferred Stock [Member]	Additional Paid-In Capital [Member]	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, 2016	$ 201,700			$ 6				$ 555,473			$ (353,673)	$ (106)
Beginning Balance, Shares at Dec. 31, 2016				58,050,280
Cumulative catch up related to the adoption of ASU | ASU 2016-09 [Member]								115			(115)
Issuance of common stock (net of commissions and offering costs), Value	152,534			$ 1				152,533
Issuance of common stock (net of commissions and offering costs), Shares				10,689,253
Exercise of stock options and vesting of restricted stock units, Value	3,281							3,281
Exercise of stock options and vesting of restricted stock units, Shares				478,471
Stock-based compensation	11,431							11,431
Issuance of shares under employee stock purchase plan, Value	677							677
Issuance of shares under employee stock purchase plan, Shares				83,687
Unrealized gain (loss) on available for sale securities	57											57
Net loss	(134,309)										(134,309)
Ending Balance, Value at Dec. 31, 2017	235,371			$ 7				723,510			(488,097)	(49)
Ending Balance, Shares at Dec. 31, 2017				69,301,691
Cumulative catch up related to the adoption of ASU | ASU 2014-09 [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							165
Stock in lieu of board fees, Shares				12,213
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

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Issuance of common stock, commissions and offering costs	$ 284	$ 260	$ 388
Royalty Pharma [Member]
Issuance of common stock, commissions and offering costs	$ 304

The Company

The Company	12 Months Ended
	Dec. 31, 2019
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 biopharmaceutical company that is committed to rewriting treatment for 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, 2019, the Company has raised, including amounts received under collaboration agreements, an aggregate of $1,280.7 million to fund its operations, of which $242.1 million was non-equity funding through its collaboration agreements, $123.1 was from funding received through agreements with RPI Finance Trust (“RPI”) and BioPharma Credit Investments V (Master) LP and BioPharma Credit PLC (the “Lenders”), $839.5 million was 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, 2019, the Company had $381.1 million in cash, cash equivalents and marketable securities. The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $757.0 million through December 31, 2019 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, 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, 2019
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 are prepared 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 FASB. The consolidated financial statements include the accounts of the Company 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 collaboration 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, 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 aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2018 was $139.2 million, which consisted of 19 commercial paper securities and 29 corporate notes securities. There were no marketable securities held by the Company for greater than twelve months as of December 31, 2018. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, mark the investment to market through a charge to the Company’s consolidated statement of operations and comprehensive loss. 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, 2019 and 2018. 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 The following table summarizes the available for sale securities held at December 31, 2018 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 73,110 $ — $ (22 ) $ 73,088 Corporate notes 80,575 — (30 ) 80,545 U.S. government agency securities and U.S. Treasuries — — — — Total $ 153,685 $ — $ (52 ) $ 153,633 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. 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. All marketable securities held at December 31, 2018 have maturities of less than one year. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2019, 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, 2019 and December 31, 2018, respectively, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. Restricted Cash As of January 1, 2018, the Company adopted ASU 2016-18, 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, 2019 2018 2017 (In thousands) Cash and cash equivalents $ 139,482 $ 86,671 $ 226,664 Restricted cash, as part of other assets 1,509 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 140,991 $ 87,133 $ 227,126 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 Financial Accounting Standards Board, or 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, 2019 and 201 8 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of December 31, 2019 and December 31, 2018 , the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 79,225 $ 50,785 $ 28,440 $ — Marketable securities: Commercial paper 73,088 — 73,088 — Corporate notes 80,545 — 80,545 — U.S. government agency securities and treasuries — — — — Total $ 232,858 $ 50,785 $ 182,073 $ — 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 cost. 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, 2019, the fair value of the long-term debt, payable in installments through November 18, 2024, approximated its carrying value due to the proximity of the issuance date to December 31, 2019. 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 N ote 11 , 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 the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans, the building of commercial infrastructure and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of December 31, 2019, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued. Accounts Receivable Accounts receivable are amounts due from collaboration partners as a result of research and development services provided, reimbursements under equally co-funded global development arrangements or milestones achieved but not yet paid. The Company considered the need for an allowance for doubtful accounts and has concluded that no allowance was needed as of December 31, 2019 or 2018, as the estimated risk of loss on its accounts receivable was determined to be minimal. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk include cash, cash equivalents, marketable securities and accounts receivable. The Company attempts to minimize the risks related to cash, cash equivalents and marketable securities by working with highly rated financial institutions that invest in a broad and diverse range of financial instruments as defined by the Company. The Company has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The Company maintains its funds in accordance with its investment policy, which defines allowable investments, specifies credit quality standards and is designed to limit the Company’s credit exposure to any single issuer. Accounts receivable represent amounts due from collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection. 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, 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 2019, 2018 or 2017. 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 6, Income Taxes Revenue Recognition Effective January 1, 2018, the Company adopted ASC, Topic 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 part ner 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 10, Collaborations Revenue Recognition Prior to Adoption of ASC 606 – Prior to the adoption of ASC 606, the Company recognized revenue when all of the following criteria were met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the Company’s price to the customer is fixed or determinable and collectability is reasonably assured. Multiple-Element Revenue Arrangements. The Company’s collaborations primarily represented multiple-element revenue arrangements. To account for these transactions, the Company determined the elements, or deliverables, included in the arrangement and allocated arrangement consideration to the various elements based on each element’s relative selling price. The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involved significant judgment, including consideration as to whether each delivered element had standalone value to the collaborator. The Company determined the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE was not available, or the Company’s best estimate of selling price, if neither VSOE nor third-party evidence was available. Determining the best estimate of selling price for a deliverable required significant judgment. The Company typically used its best estimate of a selling price to estimate the selling price for licenses to its proprietary technology, since it often did not have VSOE or third-party evidence of selling price for these deliverables. In those circumstances where the Company applied its best estimate of selling price to determine the estimated selling price of a license to its proprietary technology, it considered market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its best estimate of selling price, the Company evaluated whether changes in the key assumptions used to determine its best estimate of selling price would have a significant effect on the allocation of arrangement consideration between deliverables. The Company recognized consideration allocated to an individual element when all other revenue recognition criteria were met for that element. The Company’s multiple-element revenue arrangements generally included the following: • Exclusive Licenses . The deliverables under our collaboration agreements generally included exclusive licenses to discover, develop, manufacture and commercialize compounds with respect to one or more specified HMT targets. To account for this element of the arrangement, the Company evaluated whether the exclusive license had standalone value from the undelivered elements to the collaboration partner based on the consideration of the relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and other market participants. Arrangement consideration allocated to licenses may be recognized upon delivery of the license if facts and circumstances indicate that the license has standalone value apart from the undelivered elements, which generally include research and development services. Arrangement consideration allocated to licenses is deferred if facts and circumstances indicate that the delivered license does not have standalone value from the undelivered elements. The Company has determined that some of its exclusive licenses lack standalone value apart from the related research and development services, and in those circumstances recognized collaboration revenue from non-refundable exclusive license fees on a straight-line basis over the contracted or estimated period of performance, which is generally the period over which the research and development services are to be provided. • Research and Development Services. The deliverables under the Company’s collaboration and license agreements generally include deliverables related to research and development services to be performed on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, it recognized revenue on a gross basis for research and development services as those services were performed. • Option Arrangements. 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 at the inception of the arrangement as an upfront fee or payment, upon the exercise of an option to acquire a license or upon extending the selection period as an extension fee or payment. The accounting for option arrangements is dependent on the nature of the options granted to the collaboration partner. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the options to secure exclusive licenses. Factors that are considered in evaluating whether options are substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the options, the cost to exercise the options relative to the total upfront consideration and the

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Property and Equipment, net	3. Property and Equipment, net Property and equipment, net consists of the following: December 31, 2019 2018 (In thousands) Laboratory equipment $ 4,273 $ 4,132 Computer and office equipment, furniture 5,113 5,040 Leasehold improvements 424 414 Construction in progress 560 271 Property and equipment 10,370 9,857 Less: accumulated depreciation and amortization (8,151 ) (7,800 ) Property and equipment, net $ 2,219 $ 2,057 Depreciation and amortization expense was $0.8 million, $1.1 million and $1.6 million for the years ended December 31, 2019, 2018, and 2017, respectively.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2019
Text Block [Abstract]
Prepaid Expenses and Other Current Assets	4. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following: December 31, 2019 2018 (In thousands) Prepaid clinical and manufacturing costs $ 7,657 $ 6,295 Interest receivable on available for sale securities 943 679 Other prepaid expenses and other receivables 6,923 5,190 Total prepaid expenses and other current assets $ 15,523 $ 12,164

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following: December 31, 2019 2018 (In thousands) Employee compensation and benefits $ 7,844 $ 5,509 Research and development expenses 9,706 11,272 Professional services and other 4,999 2,919 Accrued expenses $ 22,549 $ 19,700

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	6. 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, 2019, 2018, and 2017 is as follows: 2019 2018 2017 (In thousands) Current $ (34 ) $ (127 ) $ 32 Deferred 92 184 (368 ) Total 58 57 (336 ) Income tax provision (benefit) $ 58 $ 57 $ (336 ) A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: Year Ended December 31, 2019 2018 2017 Federal statutory income tax rate 21.0 % 21.0 % 34.0 % State income taxes 6.0 5.7 4.8 Research and development and other tax credits 1.9 2.4 4.2 Permanent items (0.7 ) (0.7 ) (2.0 ) Change in valuation allowance (27.5 ) (28.3 ) 5.7 Return-to-provision adjustments — (0.1 ) (0.7 ) Rate Change — — (45.7 ) Other (0.7 ) — — Effective income tax rate 0.0 % 0.0 % 0.3 % 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, 2019 2018 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 180,877 $ 147,586 Research and development and other credit carryforwards 29,774 26,731 Capitalized start-up costs 901 1,031 Deferred revenue 1,029 1,021 Accruals and allowances 1,922 1,522 Eisai license payment 12,840 8,225 Stock compensation 6,489 4,692 Other 442 480 Sale of royalty 3,458 — Lease liability 5,990 — Gross deferred tax assets 243,722 191,288 Deferred tax asset valuation allowance (237,858 ) (191,070 ) Total deferred tax assets 5,864 218 Deferred tax liabilities: Depreciation and other (40 ) (34 ) Right of use asset (5,732 ) — Total deferred tax liabilities (5,772 ) (34 ) Net deferred tax asset (liability) $ 92 $ 184 The Company evaluated the expected recoverability of its net deferred tax assets as of December 31, 2019 and 2018, 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, 2019 and 2018, 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, 2017 and recognized as a tax benefit. As of December 31, 2018, $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 2019 tax return. As such, as of December 31, 2019, $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. As of December 31, 2019, the Company had operating loss carryforwards of approximately $665.0 million and $656.0 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 $236.5 million of the U.S. federal tax operating losses can be carried forward indefinitely. The state tax operating loss carryforwards expire commencing in 2030 and will expire at various dates through 2039. Additionally, as of December 31, 2019, the Company had research and development tax credit carryforwards of approximately $10.9 million and $3.6 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 2039, as well as federal orphan drug tax credit carryforwards of $15.9 million, which would expire at various dates from 2033 through 2038, and a $0.1 million federal alternative minimum tax credit carryforward, which represents the remaining AMT credit to be refunded with the filing of the 2020-2022 tax returns. 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, 2019 2018 (In thousands) Unrecognized tax benefits - as of beginning of year $ 5,743 $ 5,223 Gross increases - current period tax positions 585 520 Unrecognized tax benefits - as of end of year $ 6,328 $ 5,743 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, 2019 or 2018. 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, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Commitments In addition to commitments under leasing arrangements (Refer to Note 8, 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, with the remaining $0.5 million to be paid by the Company upon the satisfaction of certain conditions set forth in the fourth amendment to the companion diagnostics agreement. Through December 31, 2019, the Company has paid Roche Molecular $ 6.0 million under this amended agreement, including developmental costs of $ 4.0 million and $ 2.0 million paid in 2019 and 2018, respectively, upon the achievement of milestones under the companion diagnostics agreement with Roche Molecular. As of December 31, 2019, the Company is responsible for the r emaining development costs of $ million due under the agreement. The Company expects the remaining development costs under the amended agreement to be incurred and paid through 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, 2019
Leases [Abstract]
Leases	8. 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, (the "Technology Square Lease”), with ARE-TECH Square, LLC, a Delaware limited liability company, (the “Technology Square Landlord”), with a term that originally continued through May 31, 2018, and a Company option to extend the term of the lease at the then-current market rent, as defined in the Technology Square Lease, through November 30, 2022. In May 2017, the Company entered into a Third Amendment to the Technology Square Lease, (the “Third Amendment”), with the Technology Square Landlord, and a Fourth Amendment to the Technology Square Lease with the Technology Square Landlord, and, together with the Third Amendment, the Amendments. Under the Amendments, the Company extended the term of the Technology Square Lease to November 30, 2022 but retained the right to terminate the Technology Square Lease prior to December 31, 2017. The Company did not exercise this right. Under the Technology Square Lease as amended, the Company has 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 December 1, 2021. 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 842 transition guidance, the Company determined the classification of this lease to be operating and recorded a lease liability and a right-of-use asset on the ASC 842 effective date. On August 16, 2019, the Company entered into a lease (“the Hampshire Street Lease”) with BMR-Hampshire LLC (the “Landlord”). 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 is not payable, the Hampshire Street Lease provides for monthly rent payments starting at $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. In addition to base rent, the Hampshire Street Lease requires the Company to pay certain variable costs, including taxes, insurance, maintenance and other operating expenses. The Company has a $1.0 million letter of credit in favor of the Landlord 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 Topic 842 and other information pertaining to the Company’s operating leases for the three and nine months ended December 31, 2019: Twelve months ended December 31, 2019 Lease cost Operating lease cost $ 3,771 Variable lease cost 1,318 Total lease cost $ 5,089 Other information Operating cash flows used for operating leases $ 3,648 Weighted average remaining lease term 5.3 years Weighted average discount rate 9.60 % Future minimum lease payments under the Company’s non-cancelable operating leases as of December 31, 2019, are as follows: 2019 (In thousands) 2020 $ 4,512 2021 6,405 2022 6,189 2023 2,916 Thereafter 9,696 Total lease payments $ 29,718 Less: imputed interest (7,559 ) Total operating lease liabilities at December 31, 2019 $ 22,159 Under the prior lease accounting guidance, the Company’s contractual commitments under leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2018 were as follows: Total 2019 2020 2021 2022 2023 (In thousands) Operating leases $ 14,099 $ 3,552 $ 3,641 $ 3,574 $ 3,332 $ — Capital lease, including amounts representing interest 69 16 17 18 18 — Total commitments $ 14,168 $ 3,568 $ 3,658 $ 3,592 $ 3,350 $ —

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Stockholders' (Deficit) Equity	9. Stockholders’ (Deficit) Equity 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 stockholders are entitled to dividends when and if declared by the board of directors. In November 2019, the Company issued to RPI 6,666,667 shares of Common Stock pursuant to a purchase agreement (for additional information refer to Note 11, Sale of Future Royalties As of December 31, 2019, a total of 25,428,732 shares of common stock were reserved for issuance upon (i) the exercise of outstanding stock options and vesting of restricted stock units (ii) the issuance of stock awards under the Company’s 2013 Stock Incentive Plan and 2013 Employee Stock Purchase Plan (iii) the issuance of common stock under the Series A Preferred Stock (iv) the issuance of common stock under the warrants and (v) the issuance of common stock under the Company’s Put Option. 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 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 of $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, and determined that equity treatment was appropriate because the Series A Preferred Stock did not meet the definition of the liability instruments defined thereunder for convertible instruments. Specifically, the Series A Preferred Stock is not mandatorily redeemable and does not embody an obligation to buy back the shares outside of the Company’s control in a manner that could require the transfer of assets. Additionally, the Company determined that the Series A Preferred Stock would be recorded as permanent equity, not temporary equity, based on the guidance of ASC 480 given that the holders of equally and more subordinated equity would be entitled to also receive the same form of consideration upon the occurrence of the event that gives rise to the redemption or events of redemption that are within the control of the Company. 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. 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, warrants to purchase up to 2,500,000 shares of Common Stock at an exercise price of $20.00 per share (the “Common Stock Warrant”), were issued to RPI pursuant to the RPI Purchase Agreement (for additional information refer to Note 11, Sale of Future Royalties

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	10. Collaborations GSK In January 2011, the Company entered into a collaboration and license agreement with 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 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 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, 2019, 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, 2019, the Company has earned a total of $89.0 million under 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 year ended December 31, 2019. The Company did not have any deferred revenue related to this agreement as of December 31, 2019 or December 31, 2018 and any future revenues will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets. As of December 31, 2018, the Company had $20.0 million in receivables under this agreement which were collected in 2019. There were no receivables outstanding under this agreement as of December 31, 2019. 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 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 diagnostic agreement with Roche Molecular, and 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 Company also agreed to pay Eisai up to $20.0 million in clinical development milestone payments, including a $10.0 million milestone upon the earlier of initiation of a first phase 3 clinical trial of any EZH2 product or the first submission of an NDA or Marke t Authorization Application, (“ MAA ”) , and a $10.0 million milestone upon the earlier of initiation of a first phase 3 clinical trial of an EZH2 product or the first submission of an NDA or MAA for an indication different from the previous indication, up to $ 50.0 million in regulatory milestone payments, including a $ 25.0 million milestone payment upon regulatory approval of the first NDA or MAA, and a $25.0 million milestone payment upon regulatory approval of the next NDA or MAA of the different indication, 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 U.S. F ood and Drug Administration, (“ FDA ”) , for the treatment of patients with epithelioid sarcoma, triggering the payment of the first $ 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 follicular lymphoma, triggering the payment of the second $ 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, we triggered the payment of the $ 25.0 million milestone payment upon regulatory approval of tazemetostat for epithelioid sarcoma. Roche Molecular In December 2012, Eisai and the Company entered into an 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 entered into a second amendment to the companion diagnostic 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 has 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, with the remaining $0.5 million to be paid by the Company upon the satisfaction of certain conditions set forth in the fourth amendment to the companion diagnostics agreement. Under the agreement with Roche Molecular, Roche Molecular is obligated to use commercially reasonable efforts to develop and to make commercially available the companion diagnostic. Roche Molecular has exclusive rights to commercialize the companion diagnostic. The agreement with Roche Molecular will expire when the Company is no longer developing or commercializing tazemetostat. The Company may terminate the agreement by giving Roche Molecular 90 days’ written notice if the Company discontinues development and commercialization of tazemetostat or determines, in conjunction with Roche Molecular, that the companion diagnostic is not needed for use with tazemetostat. Either the Company or Roche Molecular may also terminate the agreement in the event of a material breach by the other 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 Molecular 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 acetyl transferase, 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, world-wide license to the undisclosed target inhibitors technology. The agreement also includes reciprocal licenses to utilize each other’s know-how, patents and technologies for activities under the agreement. Further, each party is granted the license to develop, manufacture, commercialize and otherwise exploit any compound or product that successfully achieves start of lead optimization (“SoLO”). The Company is also obligated to provide R&D services through SoLO approval for both Target Projects, and to serve on the Joint Steering Committee (“JSC”) throughout 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 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. The Company is eligible to receive up to $80.5 million in clinical development milestone payments, up to $106.5 million in regulatory milestone payments and up to $93.5 million in sales-based milestone payments. In addition, Boehringer Ingelheim is required to pay the Company tiered royalties, on a product by product, and country by country basis, at percentages ranging from the mid-single digits to low-double digits. Royalties will be payable on net product sales for therapies directed at the second target both in the United States and the rest of the world and net product sales outside of the United States for therapies directed at the first target. In the second quarter of 2019, we achieved and received a $5.5 million development milestone, which was included in the transaction price and recognized over the remaining performance period. The next potential milestone payment that the Company might be entitled to receive under this agreement is a $5.5 million milestone, for selection of a lead optimization candidate for the shared program targeting enzymes within the HAT families under the agreement. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone or royalty payments from Boehringer Ingelheim. 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 license 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; • $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 are excluded from the transaction price at inception, as the achievement of the milestone events are highly uncertain. As such, all milestone payments are fully constrained. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price. The Company determined that a 50/50 allocation of transaction price between the two performance obligations is appropriate considering the following factors: (i) R&D components’ standalone selling price estimated using the cost plus margin approach; based on cost-plus 10%; (ii) the license rights granted for each program (world-wide or ex-US only) and their potential market opportunities; (iii) the total potential milestone payments for each program; and (iv) the expected revenue recognition pattern for each program, which is expected to be relatively consistent. Therefore, $10.0 million was allocated to the first undisclosed target license and associated research services and $10.0 million was allocated to the second undisclosed target license and associated research services and was recognized through December 31, 2019. The $5.5 million development milestone for selection of a lead optimization candidate for the shared program targeting enzymes within helicase families was allocated to the first undisclosed target license and associated research services and was recognized in the year ended December 31, 2019. The allocation of the variable consideration, the development milestones, will be allocated to each performance obligation as described in the contract. The milestone payments are defined by program and are directly attributable to distinct achievements in each program. The recognition of revenue for each milestone will be based on progress to date in satisfying the applicable performance obligation. Collaboration Revenue Through December 31, 2019, the Company has recognized $25.5 million in total collaboration revenue since the inception of this collaboration, including $23.8 million during the year ended December 31, 2019, which included the recognition of $13.3 million of revenue that was deferred as of December 31, 2018. The Company did not have any deferred revenue related to this agreement as of December 31, 2019. As of December 31, 2019, the Company had $1.3 million in receivables under this agreement, which were collected in January 2020. There were no receivables outstanding under this agreement as of December 31, 2018. The Company will reevaluate the likelihood of achieving future milestones at the end of each reporting period. If the performance obligations have not been satisfied at the point at which the risk of significant revenue reversal is resolved, the transaction price will be adjusted and a cumulative catch up based on performance to date will be recorded. Celgene (a subsidiary of Bristol-Myers Squibb) In April 2012, the Company entered into a collaboration and license agreement with Celgene. On July 8, 2015, the Company entered into an amendment and restatement of the collaboration and license agreement with Celgene. 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, 2019, 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 have not yet been recognized as revenue, relate to the material rights, and are recorded in deferred revenue on the Company’s consolidated balance sheet. Deferred revenue related to the agreement amounted to $3.8 million as of December 31, 2019, all of which is included in noncurrent liabilities.

Sale of Future Royalties

Sale of Future Royalties	12 Months Ended
	Dec. 31, 2019
Deferred Revenue Disclosure [Abstract]
Sale of Future Royalties	11. Sale of Future Royalties On November 4, 2019, the Company entered into a loan agreement with the Lenders providing for up to $70.0 million in secured term loans to be advanced in up to three tranches (the “Loan Agreement”). The Company may borrow $25.0 million under each of the first two tranches and $20.0 million under the third tranche. As of December 31, 2019, the Company had borrowed an aggregate principal amount under the first tranche of $25.0 million (the “Tranche A Note Payable”), with the ability to draw down the remaining two tranches under the Loan Agreement subject to certain conditions. The Company also has the right to request up to an additional $300.0 million in secured term loans, subject to the approval of the Lenders, following FDA approval of tazemetostat for the treatment of FL in the United States, provided that the Company has not prepaid any outstanding term loans at the time of such request and such request is made before November 18, 2021. (See Note 12, Long-Term Debt On the same day, the Company 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 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 has 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”). As further discussed in Note 16, Subsequent Events 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 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 transaction was based on our current 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 warrants have been recorded in additional paid-in-capital and the Tranche A Note Payable has been recorded as long-term debt (See Note 12, Long-Term Debt Under the terms of RPI Purchase Agreement, although the Company sold all of its rights to receive the Japan Royalty, the Company continues to own all tazemetostat intellectual property rights and is responsible for the ongoing manufacturing and supply obligations 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 amortized 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 amortization 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 will periodically assess 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. The following table shows the activity of the Royalty Obligation since the transaction inception through December 31, 2019: Year Ended December 31, 2019 (In thousands ) Proceeds from sale of future royalties $ 12,601 Non-cash interest expense recognized 192 Liability related to the sale of future royalties - ending balance $ 12,793 During the year ended December 31, 2019 no non-cash royalties from net sales of tazemetostat in Japan were recorded and the Company recorded $0.2 million of related non-cash interest expense.

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Long-Term Debt	12. Long-Term Debt As of December 31, 2019, the Company had borrowed the first tranche of $25.0 million in term loans under the Loan Agreement. Under the terms of the Loan Agreement, the Company is required to make quarterly interest only payments following the closing of Tranche A Loan on November 18, 2019 and 8 equal quarterly payments of principal starting February 28, 2023 through November 18, 2024. The per annum interest rate of the Loan Agreement is equal to the LIBOR rate plus 7.75%. The Company has the ability to prepay the outstanding loan at its option by paying the greater of a prepayment penalty amount equal to the sum of all interest accruing from the prepayment date through the 36 th States, provided that the Company has not prepaid any outstanding term loans at the time of such request and such request is made before November 18, 2021. The obligations under the Loan Agreement are secured by a first priority security interest in and lien upon substantially all of the Company’s assets excluding its subsidiary, Epizyme Securities Corporation. The Loan Agreement contains negative covenants restricting the Company’s activities, including prohibition on consolidation, liquidation or dissolution, mergers or acquisitions, or change in control transactions. It also prohibits any disposition of all or any part of its properties or assets. There are no financial covenants associated with the agreement. The obligations under the agreement are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. 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 current and non-current liabilities based on scheduled principal payments. The Company has the following minimum aggregate future loan payments at December 31, 2019 (in thousands): Year Ended December 31, 2019 2020 $ — 2021 — 2022 — 2023 12,500 2024 12,500 Total minimum payments 25,000 Less amounts representing interest and discount (1,691 ) Less current portion — Long-term debt, net of current portion $ 23,309 For the year ended December 31, 2019, interest expense related to the Company's Loan Agreement was approximately $0.3 million. The total carrying value of debt is classified as long-term on the consolidated balance sheet as of December 31, 2019.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Employee Benefit Plans	13. 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 may be granted to officers, employees, consultants, advisors and other service providers. Incentive and non-qualified stock options and restricted stock granted to employees 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. 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 the recipient’s term of board service. 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 $18.0 million, $12.0 million, and $11.4 million for the years ended December 31, 2019, 2018, and 2017, respectively. Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2019 2018 2017 (In thousands) Research and development $ 6,295 $ 4,083 $ 5,613 General and administrative 11,721 7,921 5,818 Total $ 18,016 $ 12,004 $ 11,431 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, 2019 2018 2017 Risk-free interest rate 2.2 % 2.6 % 1.8 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 72.0 % 71.5 % 74.2 % 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, 2019, December 31, 2018 or 2017. 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, 2019: 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, 2018 5,153 $ 14.48 Granted 4,223 10.83 Exercised (261 ) 9.03 Forfeited or expired (1,028 ) 13.64 Outstanding at December 31, 2019 8,087 $ 12.86 7.9 $ 95,883 Exercisable at December 31, 2019 2,910 $ 15.09 6.0 $ 28,601 During the years ended December 31, 2019, 2018 and 2017, the Company granted stock options to purchase an aggregate of 4,222,693 shares, 2,537,277 shares, and 2,331,500 shares, respectively, at weighted-average grant date fair values per option share of $6.99, $9.49, and $8.85, respectively. The total grant date fair value of options that vested during the years ended December 31, 2019, 2018 and 2017 was $13.2 million, $12.1 million, and $12.0 million, respectively. The aggregate intrinsic value of stock options exercised was $1.2 million in 2019, $1.5 million in 2018 and $4.1 million in 2017. As of December 31, 2019, there was $33.6 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 2.8 years. Restricted Stock Units During the year-ended December 31, 2019, 304,960 restricted stock units (“RSUs”) were granted to executives. These 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, 2018 — $ — Granted 305 9.32 Vested — — Forfeited (21 ) 9.12 Outstanding at December 31, 2019 284 $ 9.34 Compensation expense totaling $0.5 million was recognized for the service-based RSUs to executives for the year-ended December 31, 2019. As of December 31, 2019, there was $2.1 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 3.25 years. During 2019, the Company granted approximately 604,000 RSUs to executives and employees, which contain performance conditions. 20% of the RSUs vested on June 30, 2019 and 25% of the RSUs became probable of achievement in the quarter ended December 31, 2019. The remaining performance conditions are based on regulatory approval and are not probable as of December 31, 2019. Assuming all remaining performance conditions are achieved, the Company expects that the remaining 20% would vest on March 31, 2020, and the final 30% of the RSUs would vest on September 30, 2020. Number of Performance Based RSU Shares Weighted Average Grant Date Fair Value per Unit (In thousands except per share data) Outstanding at December 31, 2018 — $ — Granted 604 12.18 Vested (95 ) 12.14 Forfeited (66 ) 12.09 Outstanding at December 31, 2019 443 $ 12.16 Compensation expense totaling $3.6 million was recognized for the performance-based RSUs for the year-ended December 31, 2019. There was $3.4 million of unrecognized compensation cost as of December 31, 2019, related to performance-based RSUs that will be recognized when and if the performance conditions become probable. As of December 31, 2019, there were approximately 727,000 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 $0.6 million, $0.5 million and $0.5 million in the years ended December 31, 2019, 2018 and 2017, respectively.

Loss per Share

Loss per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Loss per Share	14. 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, 2019 2018 2017 (In thousands except per share data) Net loss $ (170,295 ) $ (123,630 ) $ (134,309 ) Accretion of Series A Preferred Stock (2,940 ) — — Net loss attributable to common stockholders $ (173,235 ) $ (123,630 ) $ (134,309 ) Weighted average shares outstanding 89,891 71,864 61,471 Basic and diluted loss per share allocable to common stockholders $ (1.93 ) $ (1.72 ) $ (2.18 ) 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, 2019 2018 2017 (In thousands) Stock options 8,087 5,153 4,576 Restricted stock units 757 — — Shares issuable under employee stock purchase plan 38 28 23 Series A Preferred Stock (if converted) 3,500 — — Warrants 2,500 — — 14,882 5,181 4,599 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, as further Subsequent Events

Unaudited Quarterly Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Results	15. Unaudited Quarterly Results The results of operations on a quarterly basis for the years ended December 31, 2019 and 2018 are set forth below: Quarter Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 (In thousands, except per share data) Collaboration revenue $ 7,891 $ 5,900 $ 5,715 $ 4,294 Operating expenses: Research and development 26,896 40,907 26,579 38,257 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 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 Quarter Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 (In thousands, except per share data) Collaboration revenue $ — $ 12,000 $ — $ 9,700 Operating expenses: Research and development 25,622 31,346 27,027 21,838 General and administrative 9,360 10,914 11,528 12,170 Total operating expenses 34,982 42,260 38,555 34,008 Operating loss (34,982 ) (30,260 ) (38,555 ) (24,308 ) Other income, net 917 1,132 1,063 1,420 Income tax (provision) — — — (57 ) Net loss $ (34,065 ) $ (29,128 ) $ (37,492 ) $ (22,945 ) Loss per share allocable to common stockholders: Basic $ (0.49 ) $ (0.42 ) $ (0.54 ) $ (0.29 ) Diluted $ (0.49 ) $ (0.42 ) $ (0.54 ) $ (0.29 ) Weighted average shares outstanding: Basic 69,386 69,490 69,539 78,962 Diluted 69,386 69,490 69,539 78,962

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events In January 2020, we triggered the payment of the $25.0 million milestone payment upon regulatory approval by the FDA of tazemetostat for epithelioid sarcoma and subsequently paid the milestone in February 2020. On February 11, 2020, the Company sold 2,500,000 shares of Common Stock to RPI for an aggregate of $50.0 million pursuant to the Put Option.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis Of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The consolidated financial statements are prepared 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 FASB. The consolidated financial statements include the accounts of the Company 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 collaboration 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, 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 aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2018 was $139.2 million, which consisted of 19 commercial paper securities and 29 corporate notes securities. There were no marketable securities held by the Company for greater than twelve months as of December 31, 2018. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, mark the investment to market through a charge to the Company’s consolidated statement of operations and comprehensive loss. 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, 2019 and 2018. 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 The following table summarizes the available for sale securities held at December 31, 2018 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 73,110 $ — $ (22 ) $ 73,088 Corporate notes 80,575 — (30 ) 80,545 U.S. government agency securities and U.S. Treasuries — — — — Total $ 153,685 $ — $ (52 ) $ 153,633 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. 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. All marketable securities held at December 31, 2018 have maturities of less than one year. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2019, 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, 2019 and December 31, 2018, 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 As of January 1, 2018, the Company adopted ASU 2016-18, 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, 2019 2018 2017 (In thousands) Cash and cash equivalents $ 139,482 $ 86,671 $ 226,664 Restricted cash, as part of other assets 1,509 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 140,991 $ 87,133 $ 227,126 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 Financial Accounting Standards Board, or 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, 2019 and 201 8 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of December 31, 2019 and December 31, 2018 , the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 79,225 $ 50,785 $ 28,440 $ — Marketable securities: Commercial paper 73,088 — 73,088 — Corporate notes 80,545 — 80,545 — U.S. government agency securities and treasuries — — — — Total $ 232,858 $ 50,785 $ 182,073 $ — 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 cost. 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, 2019, the fair value of the long-term debt, payable in installments through November 18, 2024, approximated its carrying value due to the proximity of the issuance date to December 31, 2019.
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 N ote 11 , 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 the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans, the building of commercial infrastructure and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of December 31, 2019, the Company did not identify conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the date these financial statements were issued.
Accounts Receivable	Accounts Receivable Accounts receivable are amounts due from collaboration partners as a result of research and development services provided, reimbursements under equally co-funded global development arrangements or milestones achieved but not yet paid. The Company considered the need for an allowance for doubtful accounts and has concluded that no allowance was needed as of December 31, 2019 or 2018, as the estimated risk of loss on its accounts receivable was determined to be minimal.
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 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.
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, 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 2019, 2018 or 2017.
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 6, Income Taxes
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted ASC, Topic 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 part ner 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 10, Collaborations Revenue Recognition Prior to Adoption of ASC 606 – Prior to the adoption of ASC 606, the Company recognized revenue when all of the following criteria were met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the Company’s price to the customer is fixed or determinable and collectability is reasonably assured. Multiple-Element Revenue Arrangements. The Company’s collaborations primarily represented multiple-element revenue arrangements. To account for these transactions, the Company determined the elements, or deliverables, included in the arrangement and allocated arrangement consideration to the various elements based on each element’s relative selling price. The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involved significant judgment, including consideration as to whether each delivered element had standalone value to the collaborator. The Company determined the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE was not available, or the Company’s best estimate of selling price, if neither VSOE nor third-party evidence was available. Determining the best estimate of selling price for a deliverable required significant judgment. The Company typically used its best estimate of a selling price to estimate the selling price for licenses to its proprietary technology, since it often did not have VSOE or third-party evidence of selling price for these deliverables. In those circumstances where the Company applied its best estimate of selling price to determine the estimated selling price of a license to its proprietary technology, it considered market conditions as well as entity-specific factors, including those factors contemplated in negotiating the agreements as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its best estimate of selling price, the Company evaluated whether changes in the key assumptions used to determine its best estimate of selling price would have a significant effect on the allocation of arrangement consideration between deliverables. The Company recognized consideration allocated to an individual element when all other revenue recognition criteria were met for that element. The Company’s multiple-element revenue arrangements generally included the following: • Exclusive Licenses . The deliverables under our collaboration agreements generally included exclusive licenses to discover, develop, manufacture and commercialize compounds with respect to one or more specified HMT targets. To account for this element of the arrangement, the Company evaluated whether the exclusive license had standalone value from the undelivered elements to the collaboration partner based on the consideration of the relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and other market participants. Arrangement consideration allocated to licenses may be recognized upon delivery of the license if facts and circumstances indicate that the license has standalone value apart from the undelivered elements, which generally include research and development services. Arrangement consideration allocated to licenses is deferred if facts and circumstances indicate that the delivered license does not have standalone value from the undelivered elements. The Company has determined that some of its exclusive licenses lack standalone value apart from the related research and development services, and in those circumstances recognized collaboration revenue from non-refundable exclusive license fees on a straight-line basis over the contracted or estimated period of performance, which is generally the period over which the research and development services are to be provided. • Research and Development Services. The deliverables under the Company’s collaboration and license agreements generally include deliverables related to research and development services to be performed on behalf of the collaboration partner. As the provision of research and development services is a part of the Company’s central operations, when the Company is principally responsible for the performance of these services under the agreements, it recognized revenue on a gross basis for research and development services as those services were performed. • Option Arrangements. 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 at the inception of the arrangement as an upfront fee or payment, upon the exercise of an option to acquire a license or upon extending the selection period as an extension fee or payment. The accounting for option arrangements is dependent on the nature of the options granted to the collaboration partner. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the options to secure exclusive licenses. Factors that are considered in evaluating whether options are substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the options, the cost to exercise the options relative to the total upfront consideration and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. For arrangements under which the option to secure licenses is considered substantive, the Company did not consider the licenses to be deliverables at the inception of the arrangement. For arrangements where the option to secure licenses is not conside red substantive, the Company consider ed the license to be a deliverable at the inception of the arrangement and, upon delivery of the license, applied the multiple-element revenue arrangement criteria to the license and any other deliverables to determine the appropriate revenue recognition. None of the options to secure exclusive licenses included in our collaborative arrangements have been determined to be substantive. Milestone Revenue . The Company’s collaboration and license agreements generally include contingent milestone payments related to specified preclinical research and development milestones, clinical development milestones, regulatory milestones and sales-based milestones. Preclinical research and development milestones are typically payable upon the selection of a compound candidate for the next stage of research and development. Clinical development milestones are typically payable when a product candidate initiates or advances in clinical trial phases or achieves defined clinical events, such as proof-of-concept. Regulatory milestones are typically payable upon submission for marketing approval with regulatory authorities, upon receipt of actual marketing approvals for a compound or for additional indications or upon the first commercial sale. Sales-based milestones are typically payable when annual sales reach specified levels. At the inception of each arrangement that included milestone payments, we evaluated whether each milestone was substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation included an assessment of whether: • the consideration is commensurate with either the entity’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone; • the consideration relates solely to past performance; and • the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluated factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. Non-refundable preclinical research and development, clinical development an d regulatory milestones that were expected t o be achieved as a result of the Company’s ef forts during the period of its performance obligations under the collabor ation and license agreements were generally considered to be substantive and were recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If not considered to be substantive, revenue from achievement of milestones wa s initially deferred and recognized over the remaining term of our performance obligations. Milestones that were not cons idered substantive because the Company did not co ntribute effort to its achievement are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met, as there are no undelivered elements remaining and no continuin g performance obligations on the Company’s part.
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 1 3 , Employee Benefit Plans , for additional information regarding the measurement and recognition of expense related to the Company’s stock-based compensation awards.
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 $0.001 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 14, Loss per Share
Segment Information	Segment Information The Company operates as one reportable business segment: the discovery and development of novel epigenetic therapies for patients with cancer and other diseases.
Pending Accounting Pronouncements	Pending Accounting Pronouncements In June 2016, the FASB Financial Instruments—Credit Losses (Topic 326): Measurements of Credit Losses on Financial Instruments The FASB has subsequently issued amendments to ASU 2016-13, which will be effective for the Company January 1, 2020. These standards require that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale securities with unrealized losses, these standards now require allowances to be recorded instead of reducing the amortized cost of the investment. The Company is currently evaluating the impact of the adoption of ASU 2016-13 and does not expect adoption to have a material effect on the Company’s consolidated financial statements or disclosures. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (“ASC 808”)
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Leases In February 2016, the FASB issued ASU, 2016-02, Leases (“ASC 842”), which requires lessees to recognize a right-of-use asset and lease liability for most lease arrangements. The new standard is effective for annual reporting periods beginning after December 15, 2018. A modified retrospective transition approach is required to be applied to leases existing as of, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. Effective January 1, 2019, the Company adopted ASC 842 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 previous guidance in ASC 840, Leases (“ASC 840”). At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of 12 months or less. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless there is reasonable certainty that the Company will renew. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate, which reflects 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. In transition to ASC 842, the Company utilized the remaining lease term of its leases in determining the appropriate incremental borrowing rates. In accordance with ASC 842, components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, consumables, etc.), and non-components (e.g., property taxes, insurance, etc.). The fixed and in-substance fixed contract consideration (including any consideration related to non-components) must be allocated based on the respective relative fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain expedients are available. Entities may elect the practical expedient to not separate lease and non-lease components by class of underlying asset. Rather, entities would account for each lease component and the related non-lease component together as a single component. The Company elected to treat lease and non-lease components as a single component for leases of all underlying asset types, including real estate and non-real estate leases. In adopting ASC 842, the Company elected to utilize the available package of practical expedients permitted under the transition guidance within the new standard, which does not require the reassessment of the following: i) whether existing or expired arrangements are or contain a lease, ii) the lease classification of existing or expired leases, and iii) whether previous initial direct costs would qualify for capitalization under the new lease standard. 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. As of December 31, 2019, the Company had operating lease liabilities and right-of-use asset balances under the transition guidance within the new standard of $9.4 million and $8.7 million, respectively.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Available-for-Sale Securities Held	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 The following table summarizes the available for sale securities held at December 31, 2018 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 73,110 $ — $ (22 ) $ 73,088 Corporate notes 80,575 — (30 ) 80,545 U.S. government agency securities and U.S. Treasuries — — — — Total $ 153,685 $ — $ (52 ) $ 153,633
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, 2019 2018 2017 (In thousands) Cash and cash equivalents $ 139,482 $ 86,671 $ 226,664 Restricted cash, as part of other assets 1,509 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 140,991 $ 87,133 $ 227,126
Summary of Company's Financial Assets Recognized at Fair Value	As of December 31, 2019 and December 31, 2018 , the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2018 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 79,225 $ 50,785 $ 28,440 $ — Marketable securities: Commercial paper 73,088 — 73,088 — Corporate notes 80,545 — 80,545 — U.S. government agency securities and treasuries — — — — Total $ 232,858 $ 50,785 $ 182,073 $ —
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

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net consists of the following: December 31, 2019 2018 (In thousands) Laboratory equipment $ 4,273 $ 4,132 Computer and office equipment, furniture 5,113 5,040 Leasehold improvements 424 414 Construction in progress 560 271 Property and equipment 10,370 9,857 Less: accumulated depreciation and amortization (8,151 ) (7,800 ) Property and equipment, net $ 2,219 $ 2,057

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2019
Text Block [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: December 31, 2019 2018 (In thousands) Prepaid clinical and manufacturing costs $ 7,657 $ 6,295 Interest receivable on available for sale securities 943 679 Other prepaid expenses and other receivables 6,923 5,190 Total prepaid expenses and other current assets $ 15,523 $ 12,164

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: December 31, 2019 2018 (In thousands) Employee compensation and benefits $ 7,844 $ 5,509 Research and development expenses 9,706 11,272 Professional services and other 4,999 2,919 Accrued expenses $ 22,549 $ 19,700

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019, 2018, and 2017 is as follows: 2019 2018 2017 (In thousands) Current $ (34 ) $ (127 ) $ 32 Deferred 92 184 (368 ) Total 58 57 (336 ) Income tax provision (benefit) $ 58 $ 57 $ (336 )
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, 2019 2018 2017 Federal statutory income tax rate 21.0 % 21.0 % 34.0 % State income taxes 6.0 5.7 4.8 Research and development and other tax credits 1.9 2.4 4.2 Permanent items (0.7 ) (0.7 ) (2.0 ) Change in valuation allowance (27.5 ) (28.3 ) 5.7 Return-to-provision adjustments — (0.1 ) (0.7 ) Rate Change — — (45.7 ) Other (0.7 ) — — Effective income tax rate 0.0 % 0.0 % 0.3 %
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, 2019 2018 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 180,877 $ 147,586 Research and development and other credit carryforwards 29,774 26,731 Capitalized start-up costs 901 1,031 Deferred revenue 1,029 1,021 Accruals and allowances 1,922 1,522 Eisai license payment 12,840 8,225 Stock compensation 6,489 4,692 Other 442 480 Sale of royalty 3,458 — Lease liability 5,990 — Gross deferred tax assets 243,722 191,288 Deferred tax asset valuation allowance (237,858 ) (191,070 ) Total deferred tax assets 5,864 218 Deferred tax liabilities: Depreciation and other (40 ) (34 ) Right of use asset (5,732 ) — Total deferred tax liabilities (5,772 ) (34 ) Net deferred tax asset (liability) $ 92 $ 184
Summary of Unrecognized Tax Benefits	The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2019 2018 (In thousands) Unrecognized tax benefits - as of beginning of year $ 5,743 $ 5,223 Gross increases - current period tax positions 585 520 Unrecognized tax benefits - as of end of year $ 6,328 $ 5,743

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Summary of Lease Costs and Company's Operating Leases	The following table contains a summary of the lease costs recognized under Topic 842 and other information pertaining to the Company’s operating leases for the three and nine months ended December 31, 2019: Twelve months ended December 31, 2019 Lease cost Operating lease cost $ 3,771 Variable lease cost 1,318 Total lease cost $ 5,089 Other information Operating cash flows used for operating leases $ 3,648 Weighted average remaining lease term 5.3 years Weighted average discount rate 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, 2019, are as follows: 2019 (In thousands) 2020 $ 4,512 2021 6,405 2022 6,189 2023 2,916 Thereafter 9,696 Total lease payments $ 29,718 Less: imputed interest (7,559 ) Total operating lease liabilities at December 31, 2019 $ 22,159
Contractual Commitments	Under the prior lease accounting guidance, the Company’s contractual commitments under leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2018 were as follows: Total 2019 2020 2021 2022 2023 (In thousands) Operating leases $ 14,099 $ 3,552 $ 3,641 $ 3,574 $ 3,332 $ — Capital lease, including amounts representing interest 69 16 17 18 18 — Total commitments $ 14,168 $ 3,568 $ 3,658 $ 3,592 $ 3,350 $ —

Sale of Future Royalties (Table

Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019: Year Ended December 31, 2019 (In thousands ) Proceeds from sale of future royalties $ 12,601 Non-cash interest expense recognized 192 Liability related to the sale of future royalties - ending balance $ 12,793

Long Term Debt (Tables)

Long Term Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Schedule of Minimum Aggregate Future Loan Payments	The Company has the following minimum aggregate future loan payments at December 31, 2019 (in thousands): Year Ended December 31, 2019 2020 $ — 2021 — 2022 — 2023 12,500 2024 12,500 Total minimum payments 25,000 Less amounts representing interest and discount (1,691 ) Less current portion — Long-term debt, net of current portion $ 23,309

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 (In thousands) Research and development $ 6,295 $ 4,083 $ 5,613 General and administrative 11,721 7,921 5,818 Total $ 18,016 $ 12,004 $ 11,431
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, 2019 2018 2017 Risk-free interest rate 2.2 % 2.6 % 1.8 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 72.0 % 71.5 % 74.2 % 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, 2019: 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, 2018 5,153 $ 14.48 Granted 4,223 10.83 Exercised (261 ) 9.03 Forfeited or expired (1,028 ) 13.64 Outstanding at December 31, 2019 8,087 $ 12.86 7.9 $ 95,883 Exercisable at December 31, 2019 2,910 $ 15.09 6.0 $ 28,601
Summary of Service Based Restricted Stock Units	During the year-ended December 31, 2019, 304,960 restricted stock units (“RSUs”) were granted to executives. These 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, 2018 — $ — Granted 305 9.32 Vested — — Forfeited (21 ) 9.12 Outstanding at December 31, 2019 284 $ 9.34
Summary of Performance Based Restricted Stock Units	During 2019, the Company granted approximately 604,000 RSUs to executives and employees, which contain performance conditions. 20% of the RSUs vested on June 30, 2019 and 25% of the RSUs became probable of achievement in the quarter ended December 31, 2019. The remaining performance conditions are based on regulatory approval and are not probable as of December 31, 2019. Assuming all remaining performance conditions are achieved, the Company expects that the remaining 20% would vest on March 31, 2020, and the final 30% of the RSUs would vest on September 30, 2020. Number of Performance Based RSU Shares Weighted Average Grant Date Fair Value per Unit (In thousands except per share data) Outstanding at December 31, 2018 — $ — Granted 604 12.18 Vested (95 ) 12.14 Forfeited (66 ) 12.09 Outstanding at December 31, 2019 443 $ 12.16

Loss per Share (Tables)

Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 (In thousands except per share data) Net loss $ (170,295 ) $ (123,630 ) $ (134,309 ) Accretion of Series A Preferred Stock (2,940 ) — — Net loss attributable to common stockholders $ (173,235 ) $ (123,630 ) $ (134,309 ) Weighted average shares outstanding 89,891 71,864 61,471 Basic and diluted loss per share allocable to common stockholders $ (1.93 ) $ (1.72 ) $ (2.18 )
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, 2019 2018 2017 (In thousands) Stock options 8,087 5,153 4,576 Restricted stock units 757 — — Shares issuable under employee stock purchase plan 38 28 23 Series A Preferred Stock (if converted) 3,500 — — Warrants 2,500 — — 14,882 5,181 4,599

Unaudited Quarterly Results (Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 are set forth below: Quarter Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 (In thousands, except per share data) Collaboration revenue $ 7,891 $ 5,900 $ 5,715 $ 4,294 Operating expenses: Research and development 26,896 40,907 26,579 38,257 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 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 Quarter Ended March 31, 2018 June 30, 2018 September 30, 2018 December 31, 2018 (In thousands, except per share data) Collaboration revenue $ — $ 12,000 $ — $ 9,700 Operating expenses: Research and development 25,622 31,346 27,027 21,838 General and administrative 9,360 10,914 11,528 12,170 Total operating expenses 34,982 42,260 38,555 34,008 Operating loss (34,982 ) (30,260 ) (38,555 ) (24,308 ) Other income, net 917 1,132 1,063 1,420 Income tax (provision) — — — (57 ) Net loss $ (34,065 ) $ (29,128 ) $ (37,492 ) $ (22,945 ) Loss per share allocable to common stockholders: Basic $ (0.49 ) $ (0.42 ) $ (0.54 ) $ (0.29 ) Diluted $ (0.49 ) $ (0.42 ) $ (0.54 ) $ (0.29 ) Weighted average shares outstanding: Basic 69,386 69,490 69,539 78,962 Diluted 69,386 69,490 69,539 78,962

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basis Of Presentation [Line Items]
Issuance of common stock (net of commissions and offering costs), Value	$ 122,708	$ 81,602	$ 152,534
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 381,100
Accumulated deficit	(757,019)	$ (586,724)
Royalty Pharma [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	1,280,700
Non-equity funding through collaboration agreement	242,100
Issuance of common stock (net of commissions and offering costs), Value	78,705
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	123,100
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)	12 Months Ended
	Dec. 31, 2019USD ($)SecuritySegment	Dec. 31, 2018USD ($)Security	Dec. 31, 2017USD ($)	Jan. 01, 2019USD ($)
Accounting Policies [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 116,700,000	$ 139,200,000
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
Realized gains (losses) recognized on sale or maturity of marketable equity securities	0
Restricted cash	1,500,000
Security deposit	500,000
Allowance for doubtful accounts	0	0
Impairment of long-lived assets	$ 0	$ 0	$ 0
Largest amount of tax benefit	50.00%
Reportable business segment | Segment	1
Operating lease, liabilities	$ 22,159,000
Operating lease, right-of-use assets	21,206,000
ASU 2016-02 [Member]
Accounting Policies [Line Items]
Operating lease, liabilities	9,400,000			$ 11,500,000
Operating lease, right-of-use assets	8,700,000			$ 10,700,000
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	3	19
Corporate Notes [Member]
Accounting Policies [Line Items]
Number of securities in unrealized loss position | Security	19	29

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 241,586	$ 153,685
Available-For-Sale Securities Held, Unrealized Gains	76
Available-For-Sale Securities Held, Unrealized Losses	(57)	(52)
Available-For-Sale Securities Held, Fair Value	241,605	153,633
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	96,952	73,110
Available-For-Sale Securities Held, Unrealized Gains	27
Available-For-Sale Securities Held, Unrealized Losses	(16)	(22)
Available-For-Sale Securities Held, Fair Value	96,963	73,088
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	140,634	80,575
Available-For-Sale Securities Held, Unrealized Gains	49
Available-For-Sale Securities Held, Unrealized Losses	(41)	(30)
Available-For-Sale Securities Held, Fair Value	140,642	$ 80,545
U.S. Government Agency Securities and U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	4,000
Available-For-Sale Securities Held, Fair Value	$ 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 139,482	$ 86,671	$ 226,664
Restricted cash, as part of other assets	1,509	462	462
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 140,991	$ 87,133	$ 227,126

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, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 132,193	$ 79,225
Total	373,798	232,858
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	96,963	73,088
Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	140,642	80,545
U.S. Government Agency Securities and U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	4,000
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	124,419	50,785
Total	124,419	50,785
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	7,774	28,440
Total	249,379	182,073
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	96,963	73,088
Level 2 [Member] | Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	140,642	$ 80,545
Level 2 [Member] | U.S. Government Agency Securities and U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	$ 4,000

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	12 Months Ended
	Dec. 31, 2019
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

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment	$ 10,370	$ 9,857
Less: accumulated depreciation and amortization	(8,151)	(7,800)
Property and equipment, net	2,219	2,057
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,273	4,132
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	5,113	5,040
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	424	414
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 560	$ 271

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 840	$ 1,052	$ 1,639	[1]
[1]	Revised as a result of the adoption of ASU 2016-18

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, 2019	Dec. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid clinical and manufacturing costs	$ 7,657	$ 6,295
Interest receivable on available for sale securities	943	679
Other prepaid expenses and other receivables	6,923	5,190
Total prepaid expenses and other current assets	$ 15,523	$ 12,164

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 7,844	$ 5,509
Research and development expenses	9,706	11,272
Professional services and other	4,999	2,919
Accrued expenses	$ 22,549	$ 19,700

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, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Current			$ (34,000)	$ (127,000)	$ 32,000
Deferred			92,000	184,000	(368,000)	[1]
Total	$ 58,000	$ 57,000	58,000	57,000	(336,000)
Income tax provision (benefit)	$ 58,000	$ 57,000	$ 58,000	$ 57,000	$ (336,000)
[1]	Revised as a result of the adoption of ASU 2016-18

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
State income taxes	6.00%	5.70%	4.80%
Research and development and other tax credits	1.90%	2.40%	4.20%
Permanent items	(0.70%)	(0.70%)	(2.00%)
Change in valuation allowance	(27.50%)	(28.30%)	5.70%
Return-to-provision adjustments		(0.10%)	(0.70%)
Rate Change			(45.70%)
Other	(0.70%)
Effective income tax rate	0.00%	0.00%	0.30%

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, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 180,877	$ 147,586
Research and development and other credit carryforwards	29,774	26,731
Capitalized start-up costs	901	1,031
Deferred revenue	1,029	1,021
Accruals and allowances	1,922	1,522
Eisai license payment	12,840	8,225
Stock compensation	6,489	4,692
Other	442	480
Sale of royalty	3,458
Lease liability	5,990
Gross deferred tax assets	243,722	191,288
Deferred tax asset valuation allowance	(237,858)	(191,070)
Total deferred tax assets	5,864	218
Deferred tax liabilities:
Depreciation and other	(40)	(34)
Right of use asset	(5,732)
Total deferred tax liabilities	(5,772)	(34)
Net deferred tax asset (liability)	$ 92	$ 184

Income Taxes (Tax Cuts and Jobs

Income Taxes (Tax Cuts and Jobs Act) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	[1]
Income Tax Disclosure [Abstract]
Deferred income taxes	$ 92,000	$ 184,000	$ (368,000)
Income tax benefit	$ 0
[1]	Revised as a result of the adoption of ASU 2016-18

Income Taxes (Operating Loss Ca

Income Taxes (Operating Loss Carryforwards) - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
United States, Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 665
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	$ 236.5
Operating loss carryforwards, expiration ending year	2030
United States, State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 656
Operating loss carryforwards, expiration beginning year	2037
Operating loss carryforwards, expiration ending year	2039

Income Taxes (Research and Deve

Income Taxes (Research and Development Tax Credit) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Tax Credit Carryforward [Line Items]
Research and development tax credit carryforward expiration beginning year	2028
Research and development tax credit carryforward expiration ending year	2039
Federal orphan drug tax credit carryforwards expiration beginning year	2033
Expiry period of federal orphan drug tax credit carryforwards	2038
Federal orphan drug tax credit carryforwards	$ 15,900,000
Federal alternative minimum tax credit	100,000
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	10,900,000
Research Tax Credit Carryforward [Member] | United States, State [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	$ 3,600,000

Income Taxes - Summary of Unrec

Income Taxes - Summary of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Unrecognized tax benefits - as of beginning of year	$ 5,743	$ 5,223
Gross increases - current period tax positions	585	520
Unrecognized tax benefits - as of end of year	$ 6,328	$ 5,743

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - Roche [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2019	Mar. 31, 2018
Other Commitments [Line Items]
Regulatory milestone payments obligation			$ 1
Payments pursuant to companion diagnostic agreement	$ 6
Development costs	4	$ 2
Royalty Pharma [Member]
Other Commitments [Line Items]
Remaining development costs	4.4			$ 10.4
Reimbursements receivable of development costs	$ 0.9
Signed Amendment
Other Commitments [Line Items]
Regulatory milestone payments obligation			0.5
Fourth Amendment
Other Commitments [Line Items]
Regulatory milestone payments obligation			$ 0.5

Leases - Additional Information

Leases - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019USD ($)
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
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 (“the Hampshire Street Lease”) with BMR-Hampshire LLC (the “Landlord”). 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%
Initial Term [Member]
Lessee Lease Description [Line Items]
Lease expiration date	May 31, 2018
Extended Term [Member]
Lessee Lease Description [Line Items]
Lease expiration date	Nov. 30, 2022

Leases - Summary of Lease Costs

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

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2019USD ($)
Leases [Abstract]
2020	$ 4,512
2021	6,405
2022	6,189
2023	2,916
Thereafter	9,696
Total lease payments	29,718
Less: imputed interest	(7,559)
Total operating lease liabilities at December 31, 2019	$ 22,159

Leases - Contractual Commitment

Leases - Contractual Commitments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
Operating lease, Total	$ 14,099
Operating lease, 2019	3,552
Operating lease, 2020	3,641
Operating lease, 2021	3,574
Operating lease, 2022	3,332
Capital lease, including amounts representing interest, Total	69
Capital lease, including amounts representing interest, 2019	16
Capital lease, including amounts representing interest, 2020	17
Capital lease, including amounts representing interest, 2021	18
Capital lease, including amounts representing interest, 2022	18
Commitments, Total	14,168
Commitments, 2019	3,568
Commitments, 2020	3,658
Commitments, 2021	3,592
Commitments, 2022	$ 3,350

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Nov. 04, 2019	Mar. 06, 2019	Nov. 30, 2019	Dec. 31, 2019	Dec. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Oct. 31, 2018	Sep. 30, 2017
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 issued				97,783,000		11,500,000	79,175,000	9,583,334	10,557,000
Common stock reserved for future issuance				25,428,732
Preferred stock, shares issued				350,000			0
Net proceeds from sale of shares				$ 37,432
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
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
Number of common shares convertible for each share of convertible preferred stock				10
Percentage of conversion restriction upon holder of convertible preferred stock shares				9.99%
Conversion of stock notice period				61 days
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] | Additional Paid-In Capital [Member]
Equity [Line Items]
Proceeds from sale of equity securities fair value			$ 8,400
RPI Purchase Agreement [Member] | Maximum [Member]
Equity [Line Items]
Warrants to purchase shares of common stock	2,500,000		2,500,000

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jan. 01, 2011OptionTarget	Nov. 30, 2018USD ($)Installment	Mar. 31, 2015USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Jan. 31, 2020USD ($)	Jul. 31, 2019USD ($)	Mar. 31, 2018USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue				$ 4,294,000	$ 5,715,000	$ 5,900,000	$ 7,891,000	$ 9,700,000	$ 12,000,000		$ 23,800,000	$ 21,700,000	$ 10,000,000
Other Receivables				0				20,000,000			$ 0	20,000,000			$ 0	$ 20,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation																		$ 1,000,000
Notice period in days											90 days
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTarget	3
Clinical development milestone payment				50,000,000							$ 50,000,000				50,000,000
Additional milestone payments				197,000,000							197,000,000				197,000,000
Sales-based milestone payments				128,000,000							128,000,000				128,000,000
Additional payments received												0
Cash and accounts receivable																89,000,000
Collaboration revenue												20,000,000
Deferred revenue				0				0			0	0			0	0
GSK [Member] | Collaboration Revenue
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				20,000,000		10,000,000				10,000,000	20,000,000				20,000,000
Regulatory milestone payments obligation				50,000,000							50,000,000				50,000,000
Eisai [Member] | First 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				25,000,000
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	10,000,000				10,000,000
Eisai [Member] | Second Submission of NDA or MAA [Member] | Subsequent Event [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory milestone payments obligation																	$ 25,000,000
Royalty Pharma [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments				4,400,000							4,400,000				4,400,000				$ 10,400,000
Reimbursements receivable of development costs				900,000							900,000				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
Boehringer Ingelheim [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment received											15,000,000
Collaboration revenue		$ 25,500,000									23,800,000
Deferred revenue								13,300,000				13,300,000				13,300,000
Other Receivables				1,300,000				$ 0			1,300,000	$ 0			1,300,000	$ 0
Company received upfront payment		15,000,000
Research funding for costs to be incurred		$ 5,000,000		5,000,000							5,000,000				5,000,000
Research funding costs, payment frequency		quarterly
Research funding costs, payable installments | Installment		4
Maximum extension term of research period		1 year
Potential milestone payment		$ 5,500,000				$ 5,500,000				$ 5,500,000
Total transaction price											$ 20,000,000
Revenue, information used to determine transaction price											The Company determined that a 50/50 allocation of transaction price between the two performance obligations
Revenue, information used to allocate transaction price											(i) R&D components’ standalone selling price estimated using the cost plus margin approach; based on cost-plus 10%; (ii) the license rights granted for each program (world-wide or ex-US only) and their potential market opportunities; (iii) the total potential milestone payments for each program; and (iv) the expected revenue recognition pattern for each program, which is expected to be relatively consistent.
Research and development cost margin approach based on cost plus percentage											10.00%
Amount allocated to first undisclosed target license and associated research services											$ 10,000,000
Amount allocated to second undisclosed target license and associated research services											10,000,000
Clinical development milestone payment				5,500,000							5,500,000				5,500,000
Boehringer Ingelheim [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payment		80,500,000
Regulatory milestone payments		106,500,000
Sales-based milestone payments		$ 93,500,000
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue															99,200,000
Deferred revenue				$ 3,800,000							$ 3,800,000				$ 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, 2019USD ($)shares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	[1]	Mar. 31, 2019shares	Oct. 31, 2018shares	Sep. 30, 2017shares
Line Of Credit Facility [Line Items]
Common stock, shares issued | shares					97,783,000	79,175,000			11,500,000	9,583,334	10,557,000
Proceeds from issuance of common stock, net of commissions					$ 122,991,000	$ 81,938,000	$ 152,922,000
Royalty Purchase Agreement [Member]
Line Of Credit Facility [Line Items]
Proceeds from sale of future royalties					12,601,000
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 exercise common stock | $ / shares		$ 20		$ 20
Proceed from agreement		$ 100,000,000
Obligation to purchase common stock		50,000,000
Proceeds from issuance of common stock, net of commissions	$ 50,000,000
Proceeds from sale of future royalties					$ 12,600,000
Effective annual interest Rate					9.01%
RPI Purchase Agreement [Member] | Subsequent Event [Member]
Line Of Credit Facility [Line Items]
Shares option to sell exercised by underwriters | shares			2,500,000
Proceeds from issuance of common stock, net of commissions			$ 50,000,000
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					The Company also has the right to request up to an additional $300.0 million in secured term loans, subject to the approval of the Lenders, following FDA approval of tazemetostat for the treatment of FL in the United States, provided that the Company has not prepaid any outstanding term loans at the time of such 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					300,000,000
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] | 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
[1]	Revised as a result of the adoption of ASU 2016-18

Sale of Future Royalties - Sche

Sale of Future Royalties - Schedule of Activity of Royalty Obligation (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Deferred Revenue Arrangement [Line Items]
Non-cash interest expense recognized	$ 192
Liability related to sale of future royalties	12,793
Royalty Purchase Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Proceeds from sale of future royalties	12,601
Non-cash interest expense recognized	192
Liability related to sale of future royalties	$ 12,793

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - First Tranche [Member]	12 Months Ended
	Dec. 31, 2019USD ($)Installment
Loan Agreement [Member] | BioPharma Credit Investments V (Master) LP [Member]
Debt Instrument [Line Items]
Interest Expense Credit Facilities	$ 300,000
Loan Agreement [Member]
Debt Instrument [Line Items]
Aggregate principal amount	$ 25,000,000
Number of quarterly payments of term loan | Installment	8
Debt instrument payment first date	Feb. 28, 2023
Debt instrument payment last date	Nov. 18, 2024
Debt instrument, interest rate	7.75%
Long-term debt, maturities, repayment terms	The Company has the ability to prepay the outstanding loan at its option by paying the greater of a prepayment penalty amount equal to the sum of all interest accruing from the prepayment date through the 36th-month anniversary of the Tranche A closing date on the amount of principal prepaid
Percentage of commitment fee paid	2.00%
Commitment fee	$ 70,000,000
Loan Agreement [Member] | November 18, 2022 [Member]
Debt Instrument [Line Items]
Prepayment fee as of outstanding principal amount percentage	3.00%
Loan Agreement [Member] | Between November 18, 2022 and November 2023 [Member]
Debt Instrument [Line Items]
Prepayment fee as of outstanding principal amount percentage	2.00%
Loan Agreement [Member] | Between November 18, 2023 and November 18, 2024 [Member]
Debt Instrument [Line Items]
Prepayment fee as of outstanding principal amount percentage	1.00%
Loan Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Right to request for additional loan amount	$ 300,000,000

Long-Term Debt - Schedule of Mi

Long-Term Debt - Schedule of Minimum Aggregate Future Loan Payments (Detail) $ in Thousands	Dec. 31, 2019USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2020	$ 0
2021	0
2022	0
2023	12,500
2024	12,500
Total minimum payments	25,000
Less amounts representing interest and discount	(1,691)
Less current portion	0
Long-term debt, net of current portion	$ 23,309

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	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
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	$ 18,016	$ 12,004	$ 11,431	[1]
Non-employee stock option awards granted	0	0	0
Expected dividend yield	0.00%
Aggregate stock options granted to employees, including a non-employee	4,222,693	2,537,277	2,331,500
Weighted-average fair value of options granted	$ 6.99	$ 9.49	$ 8.85
Grant date fair value of options vested	$ 13,200	$ 12,100	$ 12,000
Aggregate intrinsic value of stock option exercised	1,200	1,500	4,100
Unrecognized compensation cost	$ 33,600
Expected weighted average period for recognition of compensation cost	2 years 9 months 18 days
Vesting period	4 years
Percentage of employer's matching contribution					50.00%
Maximum percentage of employee contribution					3.00%
Contribution to 401 (k) plan	$ 600	$ 500	$ 500
Defined contribution plan name	401(k) savings plan
Service Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	4 years
Vesting percentage	25.00%
Expected weighted average period for recognition of compensation cost	3 years 3 months
Restricted stock units outstanding	304,960
Vesting period	4 years
Compensation expense was recognized	$ 500
Unrecognized compensation cost	$ 2,100
Performance Based RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	604,000
Compensation expense was recognized	$ 3,600
Unrecognized compensation cost	$ 3,400
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	727,000
Vesting Year One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%
Vesting Remaining Years [Member]
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%
[1]	Revised as a result of the adoption of ASU 2016-18

Employee Benefit Plans - Schedu

Employee Benefit Plans - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 18,016	$ 12,004	$ 11,431	[1]
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	6,295	4,083	5,613
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 11,721	$ 7,921	$ 5,818
[1]	Revised as a result of the adoption of ASU 2016-18

Employee Benefit Plans - Assump

Employee Benefit Plans - Assumptions Used in Applying Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	2.20%	2.60%	1.80%
Expected life of options	6 years	6 years	6 years
Expected volatility of underlying stock	72.00%	71.50%	74.20%
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	5,153,000
Number of Options, Granted	4,222,693	2,537,277	2,331,500
Number of Options, Exercised	(261,000)
Number of Options, Forfeited or expired	(1,028,000)
Number of Options, Outstanding, Ending balance	8,087,000	5,153,000
Number of Options, Exercisable	2,910,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 14.48
Weighted Average Exercise Price per Share, Granted	10.83
Weighted Average Exercise Price per Share, Exercised	9.03
Weighted Average Exercise Price per Share, Forfeited or expired	13.64
Weighted Average Exercise Price per Share, Outstanding, Ending balance	12.86	$ 14.48
Weighted Average Exercise Price per Share, Exercisable	$ 15.09
Weighted Average Remaining Contractual Term (In Years), Outstanding	7 years 8 months 12 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding	$ 95,883
Aggregate Intrinsic Value, Exercisable	$ 28,601

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, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Outstanding Shares, Beginning Balance | shares	0
Number of Shares, Granted | shares	304,960
Number of Shares, Forfeited | shares	(21,000)
Number of Outstanding Shares, Ending Balance | shares	284,000
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 0
Weighted Average Grant Date Fair Value, Granted | $ / shares	9.32
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	9.12
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 9.34

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, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Outstanding Shares, Beginning Balance | shares	0
Number of Shares, Granted | shares	604
Number of Shares, Vested | shares	(95)
Number of Shares, Forfeited | shares	(66)
Number of Outstanding Shares, Ending Balance | shares	443
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 0
Weighted Average Grant Date Fair Value, Granted | $ / shares	12.18
Weighted Average Grant Date Fair Value, Vested | $ / shares	12.14
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	12.09
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 12.16

Loss per Share - Additional Inf

Loss per Share - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	[1]	Nov. 30, 2019	Nov. 04, 2019	Oct. 31, 2018	Sep. 30, 2017
Accretion of series A convertible preferred stock		$ 2,940	$ 2,940
Common stock, shares issued		11,500,000	97,783,000	79,175,000					9,583,334	10,557,000
Proceeds from issuance of common stock, net of commissions			$ 122,991	$ 81,938	$ 152,922
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
RPI Purchase Agreement [Member] | Maximum [Member]
Additional shares excluded from the additional option to sell			6,250,000
[1]	Revised as a result of the adoption of ASU 2016-18

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net loss	$ (56,402)	$ (36,089)	$ (48,465)	$ (29,339)	$ (22,945)	$ (37,492)	$ (29,128)	$ (34,065)	$ (170,295)	$ (123,630)	$ (134,309)
Accretion of convertible preferred stock				(2,940)					(2,940)
Net loss attributable to common stockholders	$ (56,402)	$ (36,089)	$ (48,465)	$ (32,279)					$ (173,235)	$ (123,630)	$ (134,309)
Weighted average shares outstanding									89,891	71,864	61,471
Basic and diluted loss per share allocable to common stockholders									$ (1.93)	$ (1.72)	$ (2.18)

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	14,882	5,181	4,599
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	8,087	5,153	4,576
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	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	38	28	23
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	3,500
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

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 4,294	$ 5,715	$ 5,900	$ 7,891	$ 9,700		$ 12,000		$ 23,800	$ 21,700	$ 10,000
Type of Revenue [Extensible List]	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember
Operating expenses:
Research and development	$ 38,257	$ 26,579	$ 40,907	$ 26,896	$ 21,838	$ 27,027	$ 31,346	$ 25,622	$ 132,639	$ 105,833	$ 109,661
General and administrative	23,530	17,089	15,698	11,986	12,170	11,528	10,914	9,360	68,303	43,972	37,181
Total operating expenses	61,787	43,668	56,605	38,882	34,008	38,555	42,260	34,982	200,942	149,805	146,842
Operating loss	(57,493)	(37,953)	(50,705)	(30,991)	(24,308)	(38,555)	(30,260)	(34,982)	(177,142)	(128,105)	(136,842)
Other income, net	1,149	1,864	2,240	1,652	1,420	1,063	1,132	917	6,905	4,532	2,197
Income tax (provision) benefit	(58)				(57)				(58)	(57)	336
Net loss	(56,402)	(36,089)	(48,465)	(29,339)	(22,945)	(37,492)	(29,128)	(34,065)	(170,295)	(123,630)	(134,309)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(56,402)	(36,089)	(48,465)	(29,339)	$ (22,945)	$ (37,492)	$ (29,128)	$ (34,065)	(170,295)	(123,630)	(134,309)
Accretion of convertible preferred stock				(2,940)					(2,940)
Net loss attributable to common stockholders	$ (56,402)	$ (36,089)	$ (48,465)	$ (32,279)					$ (173,235)	$ (123,630)	$ (134,309)
Loss per share allocable to common stockholders:
Basic	$ (0.59)	$ (0.40)	$ (0.53)	$ (0.39)	$ (0.29)	$ (0.54)	$ (0.42)	$ (0.49)	$ (1.93)	$ (1.72)	$ (2.18)
Diluted	$ (0.59)	$ (0.40)	$ (0.53)	$ (0.39)	$ (0.29)	$ (0.54)	$ (0.42)	$ (0.49)	$ (1.93)	$ (1.72)	$ (2.18)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders:
Basic	95,074	91,044	90,876	82,424	78,962	69,539	69,490	69,386	89,891	71,864	61,471
Diluted	95,074	91,044	90,876	82,424	78,962	69,539	69,490	69,386	89,891	71,864	61,471

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ in Thousands	Feb. 11, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 29, 2020
Subsequent Event [Line Items]
Issuance of common stock (net of commissions and offering costs), Value		$ 122,708	$ 81,602	$ 152,534
Forecast [Member]
Subsequent Event [Line Items]
Regulatory milestone payments obligation					$ 25,000
RPI Purchase Agreement [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Issuance of common stock (net of commissions and offering costs), Shares	2,500,000
Issuance of common stock (net of commissions and offering costs), Value	$ 50,000