Epizyme (EPZM) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 19, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1,571,498
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
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		79,206,698
Entity Public Float			$ 753.2

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 86,671	$ 226,664
Marketable securities	153,633	49,775
Accounts receivable	20,067	382
Prepaid expenses and other current assets (Note 4)	12,164	8,983
Total current assets	272,535	285,804
Property and equipment, net (Note 3)	2,057	2,527
Restricted cash and other assets	909	1,028
Total assets	275,501	289,359
Current liabilities:
Accounts payable	4,780	7,001
Accrued expenses (Note 5)	19,700	17,549
Current portion of capital lease obligation	16	110
Current portion of deferred revenue	13,300
Other current liabilities	37	4
Total current liabilities	37,833	24,664
Capital lease obligation, net of current portion	53
Deferred revenue	3,806	28,809
Other long-term liabilities	800	515
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; no shares issued and outstanding
Common stock, $0.0001 par value; 125,000 shares authorized; 79,175 shares and 69,302 shares issued and outstanding, respectively	8	7
Additional paid-in capital	819,779	723,510
Accumulated other comprehensive loss	(54)	(49)
Accumulated deficit	(586,724)	(488,097)
Total stockholders’ equity	233,009	235,371
Total liabilities and stockholders’ equity	$ 275,501	$ 289,359

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
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	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	79,175,000	69,302,000
Common stock, shares outstanding	79,175,000	69,302,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 21,700	$ 10,000	$ 8,007
Type of Revenue [Extensible List]	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember
Operating expenses:
Research and development	$ 105,833	$ 109,661	$ 91,461
General and administrative	43,972	37,181	28,372
Total operating expenses	149,805	146,842	119,833
Operating loss	(128,105)	(136,842)	(111,826)
Other income, net:
Interest income, net	4,557	2,165	1,531
Other (expense) income, net	(25)	32	83
Other income, net	4,532	2,197	1,614
Loss before income taxes	(123,573)	(134,645)	(110,212)
Income tax (provision) benefit	(57)	336
Net loss	(123,630)	(134,309)	(110,212)
Other comprehensive loss:
Unrealized (loss) gain on available for sale securities	(5)	57	(106)
Comprehensive loss	$ (123,635)	$ (134,252)	$ (110,318)
Loss per share allocable to common stockholders:
Basic	$ (1.72)	$ (2.18)	$ (1.93)
Diluted	$ (1.72)	$ (2.18)	$ (1.93)
Weighted average shares outstanding:
Basic	71,864	61,471	57,126
Diluted	71,864	61,471	57,126

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss		$ (123,630,000)	$ (134,309,000)	[1]	$ (110,212,000)	[1]
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		1,052,000	1,639,000	[1]	1,589,000	[1]
Stock-based compensation		12,004,000	11,431,000	[1]	10,568,000	[1]
Amortization of discount on investments		(1,556,000)	265,000	[1]	(51,000)	[1]
Deferred income taxes		184,000	(368,000)	[1]
Changes in operating assets and liabilities:
Accounts receivable		(19,686,000)	(359,000)	[1]	239,000	[1]
Prepaid expenses and other current assets		(3,181,000)	(2,525,000)	[1]	(1,542,000)	[1]
Accounts payable		(2,404,000)	1,967,000	[1]	341,000	[1]
Accrued expenses		2,066,000	1,523,000	[1]	4,673,000	[1]
Deferred revenue		13,300,000			(1,900,000)	[1]
Other assets		(66,000)	(17,000)	[1]	106,000	[1]
Other long-term liabilities		317,000	320,000	[1]	(182,000)	[1]
Net cash used in operating activities		(121,600,000)	(120,433,000)	[1]	(96,371,000)	[1]
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities		(298,670,000)	(126,356,000)	[1]	(229,887,000)	[1]
Proceeds from sales/maturities of marketable securities		196,363,000	240,670,000	[1]	65,097,000	[1]
Purchases of property and equipment		(299,000)	(984,000)	[1]	(624,000)	[1]
Net cash (used in) provided by investing activities		(102,606,000)	113,330,000	[1]	(165,414,000)	[1]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from public offering of common stock, net of commissions		81,938,000	152,922,000	[1]	130,438,000	[1]
Payment of common stock offering costs		(260,000)	(388,000)	[1]	(483,000)	[1]
Payment under capital lease obligation		(129,000)	(620,000)	[1]	(561,000)	[1]
Proceeds from stock options exercised		1,885,000	3,281,000	[1]	1,589,000	[1]
Issuance of shares under employee stock purchase plan		779,000	677,000	[1]	374,000	[1]
Net cash provided by financing activities		84,213,000	155,872,000	[1]	131,357,000	[1]
Net (decrease) increase in cash and cash equivalents		(139,993,000)	148,769,000	[1]	(130,428,000)	[1]
Cash, cash equivalents, and restricted cash, beginning of period	[1]	227,126,000	78,357,000		208,785,000
Cash, cash equivalents, and restricted cash, end of period		87,133,000	227,126,000	[1]	$ 78,357,000	[1]
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Unpaid offering costs		75,000
Property and equipment included in accounts payable or accruals		194,000	58,000	[1]
Cash paid for income taxes		$ 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	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance, Value at Dec. 31, 2015	$ 169,532	$ 4	$ 412,989	$ (243,461)
Beginning Balance, Shares at Dec. 31, 2015		41,785,774
Issuance of common stock (net of commissions and offering costs), Value	129,955	$ 2	129,953
Issuance of common stock (net of commissions and offering costs), Shares		15,420,220
Exercise of stock options, Value	1,589		1,589
Exercise of stock options, Shares		788,097
Stock-based compensation	10,568		10,568
Issuance of shares under employee stock purchase plan, Value	374		374
Issuance of shares under employee stock purchase plan, Shares		56,189
Unrealized gain (loss) on available for sale securities	(106)				$ (106)
Net loss	(110,212)			(110,212)
Ending Balance, Value at Dec. 31, 2016	201,700	$ 6	555,473	(353,673)	(106)
Ending 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
Exercise of stock options, Shares	215,000
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

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Stockholders Equity [Abstract]
Issuance of common stock, commissions and offering costs	$ 260	$ 388	$ 483

The Company

The Company	12 Months Ended
	Dec. 31, 2018
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 late-stage 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. The Company is developing its lead product candidate, tazemetostat, an oral, first-in-class selective inhibitor of the EZH2 histone methyltransferase, or HMT, in a broad range of cancer types and settings, and developing its lead development candidate in its novel G9a program, EZM8266, for the treatment of sickle cell disease, or SCD. Through December 31, 2018, the Company has raised, including amounts received under collaboration agreements, an aggregate of $988.2 million to fund its operations, of which $232.8 million was non-equity funding through its collaboration agreements, $679.4 million was from the sale of common 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, 2018, the Company had $240.3 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 $586.7 million through December 31, 2018 and will require substantial additional capital to fund its research and development. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risks of failure of 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, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation 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 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 within the consolidated balance sheets. 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, 2018 was $139.2 million, which consisted of 19 commercial paper securities and 29 corporate notes securities. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2017 was $42.8 million, which consisted of 4 commercial paper securities, 14 corporate notes securities, and 1 U.S. government agency security. 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 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, 2018 and 2017. 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 The following table summarizes the available for sale securities held at December 31, 2017 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 16,964 $ — $ (6 ) $ 16,958 Corporate notes 31,610 — (43 ) 31,567 U.S. government agency securities and U.S. Treasuries 1,250 — — 1,250 Total $ 49,824 $ — $ (49 ) $ 49,775 Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. All marketable securities held at December 31, 2018 and 2017 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, 2018, 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, 2018 and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. The aggregate fair value of available-for-sale securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2018 was $139.2 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of December 31, 2018 was less than $0.1 million. The Company determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2018. The weighted-average maturity of the Company’s portfolio was approximately two months at December 31, 2018. 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, 2018 and 2017 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of December 31, 2018 and December 31, 2017, the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 207,251 $ 207,251 $ — $ — Marketable securities: Commercial paper 16,958 — 16,958 — Corporate notes 31,567 — 31,567 — U.S. government agency securities and treasuries 1,250 — 1,250 — Total $ 257,026 $ 207,251 $ 49,775 $ — 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. 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. Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of December 31, 2018, 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, 2018 or 2017, 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 5 - 10 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 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 2018, 2017 or 2016. 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 On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”). This legislation makes broad and complex changes to the U.S. tax code, including, but not limited to, (i) reducing the U.S. federal statutory tax rate from 35% to 21%; (ii) eliminating the corporate alternative minimum tax (AMT) and changing how existing AMT credits can be realized; (iii) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017, (iv) modifying the officer’s compensation limitation, (v) changing rules related to the deductibility of entertainment expenses beginning in 2018, and (vi) changing rules related to the deductibility of qualified transportation benefits beginning in 2018. The Company recognizes the effects of changes in tax law, including the TCJA, in the period the law is enacted. Accordingly, the effects of the TCJA have been recognized in the financial statements as applicable for the years ended December 31, 2017 and December 31, 2018. In December 2017, the SEC staff issued Staff Accounting Bulletin, or SAB, No. 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of TCJA. As of December 31, 2017, the effects of the TCJA were recorded on a provisional basis. As of December 31, 2018, the Company finalized its accounting for the TCJA and no measurement adjustments were recorded. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification, or 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 partner related to the licensed targets. Payments to the Company under these agreements may include non-refundable license fees, customer option exercise fees, payments for research activities, reimbursement of certain costs, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company first evaluates license and/or collaboration arrangements to determine whether the arrangement (or part of the arrangement) represents a collaborative arrangement pursuant to ASC Topic 808, Collaborative Arrangements, , which represent a collaborative relationship and not a customer relationship, Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses – If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promises, whether the value of the license is dependent on the unsatisfied promises, whether there are other vendors that could provide the remaining promises, and whether it is separately identifiable from the remaining promises. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services – The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under above. Reimbursements from the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options – The Company’s arrangements may provide a collaborator with the right to select a target for licensing either at the inception of the arrangement or within an initial pre-defined selection period, which may, in certain cases, include the right of the collaborator to extend the selection period. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires. Milestone Payments – At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Royalties – For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 9, 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. The Company has entered into collaboration and license agreements to discover, develop, manufacture and commercialize compounds directed to specific HMT targets. The terms of these agreements typically contain multiple deliverables, which may include: (i) licenses, or options to obtain licenses, to compounds directed to specific HMT targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed HMT targets. Payments to the Company under these agreements may include non-refundable license fees, option fees, exercise fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. 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. Our 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, we 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. We have determined that some of our exclusive licenses lack standalone value apart from the related research and development services, and in those circumstances we 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 our 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 our central operations, when we are principally responsible for the performance of these services under the agreements, we recognized revenue on a gross basis for research and development services as those services were performed. • Option Arrangements. Our 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 us 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, we are 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, we did not consider the licenses to be deliverables at the inception of the arrangement. For arrangements where the option to secure licenses is not considered substantive, we considered 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 . Our collab

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment, net	3. Property and Equipment, net Property and equipment, net consists of the following: December 31, 2018 2017 (In thousands) Laboratory equipment $ 4,132 $ 4,138 Computer and office equipment, furniture (1) 5,040 4,807 Leasehold improvements 414 354 Construction in progress 271 71 Property and equipment 9,857 9,370 Less: accumulated depreciation and amortization (7,800 ) (6,843 ) Property and equipment, net $ 2,057 $ 2,527 (1) In 2015, the Company acquired $1.7 million in computer hardware and equipment, pursuant to a capital lease, the term of which expires in February 2018. In April 2018, the Company acquired $0.1 million of equipment pursuant to a capital lease, the term of which expires in April 2023. Accumulated depreciation related to these assets totaled $0.0 million and $1.6 million as of December 31, 2018 and 2017, respectively. Depreciation and amortization expense was $1.1 million, $1.6 million and $1.6 million for the years ended December 31, 2018, 2017, and 2016, respectively.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 (In thousands) Prepaid clinical and manufacturing costs $ 6,295 $ 5,724 Interest receivable on available for sale securities 679 249 Other prepaid expenses and other receivables 5,190 3,010 Total prepaid expenses and other current assets $ 12,164 $ 8,983

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following: December 31, 2018 2017 (In thousands) Employee compensation and benefits $ 5,509 $ 4,628 Research and development expenses 11,272 11,658 Professional services and other 2,919 1,263 Accrued expenses $ 19,700 $ 17,549

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
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, 2018, 2017, and 2016 is as follows: 2018 2017 2016 (In thousands) Current $ (127 ) $ 32 $ — Deferred 184 (368 ) — Total 57 (336 ) — Income tax provision (benefit) $ 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, 2018 2017 2016 Federal statutory income tax rate 21.0 % 34.0 % 34.0 % State income taxes 5.7 4.8 4.5 Research and development and other tax credits 2.4 4.2 3.3 Permanent items (0.7 ) (2.0 ) (1.9 ) Change in valuation allowance (28.3 ) 5.7 (40.1 ) Return-to-provision adjustments (0.1 ) (0.7 ) — Change in deferred taxes — — 0.2 Rate Change — (45.7 ) — Effective income tax rate 0.0 % 0.3 % 0.0 % Deferred Tax Assets (Liabilities) The Company’s deferred tax assets (liabilities) included in other assets in the consolidated balance sheets consist of the following: December 31, 2018 2017 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 147,586 $ 116,869 Research and development and other credit carryforwards 26,731 24,257 Capitalized start-up costs 1,031 1,175 Deferred revenue 1,021 7,766 Accruals and allowances 1,522 1,250 Eisai license payment 8,225 8,985 Other 5,172 4,738 Gross deferred tax assets 191,288 165,040 Deferred tax asset valuation allowance (191,070 ) (164,607 ) Total deferred tax assets 218 433 Deferred tax liabilities: Depreciation and other (34 ) (65 ) Total deferred tax liabilities (34 ) (65 ) Net deferred tax asset (liability) $ 184 $ 368 The Company evaluated the expected recoverability of its net deferred tax assets as of December 31, 2018 and 2017, and determined that, with the exception of the deferred tax asset related to alternative minimum tax, or 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, 2018 and 2017, 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 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. Fifty percent of the deferred tax asset related to the AMT Credit is refundable with the filing of the 2018 tax return. As such, as of December 31, 2018, $184,000 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 balance sheet. As a result of the enacted law, the Company was required to revalue deferred tax assets and liabilities at 21 percent as of December 31, 2017. This revaluation resulted in a provision of $61.6 million to income tax expense in continuing operations related to the reduction in the carrying value of the Company’s deferred tax assets, offset by a corresponding reduction in the valuation allowance in 2017. As of December 31, 2018, the Company had operating loss carryforwards of approximately $542.6 million and $535.4 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 $114.1 million of the U.S. federal tax operating losses can be carried forward indefinitely. The state tax operating loss carryforwards expire commencing in 2030. Additionally, as of December 31, 2018, the Company had research and development tax credit carryforwards of approximately $9.1 million and $2.9 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 2038, as well as federal orphan drug tax credit carryforwards of $15.2 million, which would expire at various dates from 2033 through 2038, and a $0.2 million federal alternative minimum tax credit carryforward, which represents the remaining AMT credit to be refunded with the filing of the 2019-2022 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, 2018 2017 (In thousands) Unrecognized tax benefits - as of beginning of year $ 5,223 $ 4,206 Gross increases - current period tax positions 520 1,017 Unrecognized tax benefits - as of end of year $ 5,743 $ 5,223 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, 2018 or 2017. The Company files income tax returns in the U.S. federal tax jurisdiction and Colorado, Indiana, Massachusetts, and North Carolina 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. In December 2017, the SEC staff issued SAB 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of TCJA. The Company did not record any adjustments in the year ended December 31, 2018 to provisional amounts that were material to its financial statements. As of December 31, 2018, the Company’s accounting treatment is complete.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments 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 “Lease”) with ARE-TECH Square, LLC, a Delaware limited liability company (the “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 Lease, through November 30, 2022. In May 2017, the Company entered into a Third Amendment to Lease (the “Third Amendment”) with the Landlord, and a Fourth Amendment to Lease with the Landlord (the “Fourth Amendment,” and, together with the Third Amendment, the “Amendments”). Under the Amendments, the Company extended the term of the lease to November 30, 2022 but retained the right to terminate the Lease effective as of December 31, 2018, by giving written notice to the Landlord by December 31, 2017 and paying an early termination fee. The Company did not exercise this right. Under the 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 this lease and has recorded cash held to secure this letter of credit as restricted cash in restricted cash and other assets on the consolidated balance sheet. The Company recognizes rent expense, including escalation charges, on a straight-line basis over the initial term of the lease agreement. In addition, the Company has a capital lease related to computer hardware equipment, an operating lease for storage space in Colorado and an operating lease for office space in North Carolina. Rent expense was $3.5 million, $3.3 million and $2.8 million for the years ended December 31, 2018, 2017, and 2016, respectively. The Company’s contractual commitments under these leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2018 are 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 $ — In addition to commitments under leasing arrangements, as of December 31, 2018, the Company has committed to $8.4 million of remaining development costs payable to Roche Molecular Systems Inc, (“Roche Molecular”) upon certain development and regulatory milestones, under the amended companion diagnostic agreement, and Eisai has agreed to reimburse the Company $0.9 million of this amount related to a regulatory milestone for Japan. The Company expects the remaining development costs under the amended agreement to be incurred and paid through 2020. Developmental costs of $2.0 million and $1.5 million were paid in 2018 and 2017, respectively, upon the achievement of milestones under the companion diagnostic agreement with Roche Molecular. In addition, the contractual commitments table above does not include potential future milestones or royalties that the Company may be required to make under license and collaboration agreements, including potential future milestones or royalties payable to Eisai under the amended collaboration and license agreement, due to the uncertainty of the occurrence of the events requiring payment under these agreements. Refer to Note 9, Collaborations 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 the contractual commitments table above.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' (Deficit) Equity	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the board of directors. In October 2018, September 2017 and January 2016, the Company issued 9,583,334, 10,557,000 and 15,333,334 shares of Common Stock, respectively, in connection with public offerings. The issuance of these shares contributed to significant increases in the Company’s shares outstanding as of December 31, 2018 and 2017 and in the weighted average shares outstanding for the years ended December 31, 2018 and 2017 when compared to the comparable prior year periods. The Company sold 132,253 shares and 23,581 shares, of Common Stock during the years ended December 31, 2017 and December 31, 2016, respectively, under an “at the market” program (“ATM Facility”) with Cowen and Company, LLC (“Cowen”) acting as sales agent under a sales agreement that the Company and Cowen entered into in April 2016. Cowen was compensated at a fixed commission rate of 3.0%. Transactions under the ATM Facility resulted in net proceeds of $1.6 million and $0.3 million in the years ended December 31, 2017 and 2016, respectively. The Company also incurred other issuance related costs of $0.1 million associated with the ATM Facility in the fourth quarter of 2017, which have been accounted for as an offset to additional paid in capital. Through March 10, 2017, including sales in the year ended December 31, 2017, the Company sold 155,834 shares of Common Stock under the sales agreement with Cowen. The Company terminated the sales agreement with Cowen, effective March 10, 2017. As of December 31, 2018, a total of 11,090,646 shares of common stock were reserved for issuance upon (i) the exercise of outstanding stock options and vesting of restricted stock units and (ii) the issuance of stock awards under the Company’s 2013 Stock Incentive Plan and 2013 Employee Stock Purchase Plan.

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	Celgene 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. Original Agreement Structure Under the original agreement, the Company granted Celgene an exclusive license, for all countries other than the United States, to small molecule HMT inhibitors targeting the DOT1L HMT, including pinometostat, and an option, on a target-by-target basis, to exclusively license, for all countries other than the United States, rights to small molecule HMT inhibitors targeting any HMT targets, other than the EZH2 HMT, including tazemetostat, and targets covered by the Company’s collaboration and license agreement dated January 8, 2011 with GlaxoSmithKline, or GSK. Under the original agreement, Celgene’s option was exercisable during an option period that would have expired on July 9, 2015. Under the original agreement, the Company received a $65.0 million upfront payment and $25.0 million from the sale of its series C redeemable convertible preferred stock to an affiliate of Celgene, of which $3.0 million was considered a premium and included as collaboration arrangement consideration for a total upfront payment of $68.0 million. In addition, the Company has received a $25.0 million clinical development milestone payment and $7.0 million in global development co-funding through December 31, 2018. The Company was also eligible to receive $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L as well as up to $65.0 million in payments, including a combination of clinical development milestone payments and an option exercise fee for each available target to which Celgene had the right to exercise its option during an initial option period that would have ended in July 2015 but was extended pursuant to the amended and restated agreement as discussed below under “Amended and Restated Agreement Structure” (each a “selected target”), and up to $100.0 million in regulatory milestone payments for each selected target. As to DOT1L and each selected target, the Company retained all product rights in the United States and was eligible to receive royalties for each target at defined percentages ranging from the mid-single digits to the mid-teens on net product sales outside of the United States subject to reduction in specified circumstances. The Company was obligated to conduct and solely fund research and development costs of the Phase 1 clinical trials for pinometostat. For all remaining DOT1L program development costs, Celgene and the Company were to equally co-fund global development and each party was to solely fund territory-specific development costs for its territory. Amended and Restated Agreement Structure Under the amended and restated collaboration and license agreement: • Celgene retained its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, • Celgene’s other option rights were narrowed to small molecule HMT inhibitors targeting three predefined targets (the “Option Targets”), • The exclusive licenses to HMT inhibitors targeting two of the Option Targets that Celgene may acquire were expanded to include the United States, with the exclusive license to HMT inhibitors targeting the third Option Target continuing to be for all countries other than the United States, • Celgene’s option period was extended for each of the Option Targets and Celgene’s option is exercisable at the time of the Company’s investigational new drug application, or IND, filing for an HMT inhibitor targeting the applicable Option Target, upon the payment by Celgene at such time of a pre-specified development milestone-based license payment, • Celgene’s license may be maintained beyond the end of Phase 1 clinical development for each of the Option Targets, upon payment by Celgene at such time of a pre-specified development milestone-based license payment, and • The Company’s research and development obligations with respect to each Option Target under the amended and restated agreement were extended for at least an additional three years, subject to Celgene exercising its option with respect to such Option Target at IND filing. Subject to the Company’s opt-out rights, the Company’s research and development obligations were expanded to include the completion of a Phase 1 clinical trial as to each Option Target following Celgene’s exercise of its option at IND filing. Under the amended and restated agreement, the Company received a $10.0 million upfront payment in exchange for the Company’s extension of Celgene’s option rights to the Option Targets and the Company’s research and development obligations. In addition, the Company is eligible to earn an aggregate of up to $75.0 million in development milestones and license payments, up to $365.0 million in regulatory milestone payments and up to $170.0 million in sales milestone payments related to the three Option Targets. The Company is also eligible to receive royalties on each of the Option Targets as specified in the amended and restated agreement. The Company is also eligible to earn $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L. 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 payments or royalty payments from Celgene. Due to the varying stages of development of each target, the Company is not able to determine the next milestone that might be earned, if any. The amended and restated agreement eliminated the right of first negotiation that the Company had granted to Celgene under the original agreement with respect to business combination transactions that the Company may desire to pursue with third parties. The Company is primarily responsible for the research strategy under the collaboration. During each applicable option period the Company is required to use commercially reasonable efforts to carry out a mutually agreed-upon research plan for each Option Target. Subject to the Company’s opt-out right, for the DOT1L target and each of the Option Targets, the Company is required to conduct and solely fund development costs of the Phase 1 clinical trials for HMT inhibitors directed to such targets, including for pinometostat. After the completion of Phase 1 development, as to DOT1L and the Option Target for which the Company retains U.S. rights, Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its respective territory; and, as to the other two Option Targets, after the completion of Phase 1 development, Celgene will solely fund all development costs on a worldwide basis. Accounting Considerations of the Amended and Restated Agreement The Company assessed the amended arrangement in accordance with ASC 606 and concluded that the contract counterparty, Celgene, is a customer based on the arrangement structure, through the satisfaction of each target’s performance obligations. As of the amendment, the Company identified the following performance obligations under the arrangement, whether satisfied or not: • an exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, combined with pre-IND research services for DOT1L; • post-IND research and development services for DOT1L through a Phase 1 clinical trial; • pre-IND research services for each Option Target; and • material rights related to each of Celgene’s options at the time of an IND filing to license HMT inhibitors targeting each Option Target. The Company determined that the DOT1L license and pre-IND research and development activities for DOT1L were not distinct from one another, due to the limited economic benefit that Celgene would derive from the DOT1L license if it did not obtain the research services After IND effectiveness, the Company concluded that the DOT1L license would be distinct apart from any remaining research and development services because Celgene, or other market participants, would have the ability to execute human clinical trials on the identified compound. Accordingly, the DOT1L license and pre-IND research services for DOT1L were accounted for as a combined performance obligation. The post-IND research and development services for DOT1L have been accounted for as a separate performance obligation. The pre-IND research services for each Option Target were the only performance obligations not subject to the exercise of a customer option at the time of the amendment for each Option Target and therefore represent three separate performance obligations (one for each Option Target). The Company evaluated the option rights at the time of an IND filing to determine whether they provide Celgene with material rights. The Company concluded that the options were issued at a discount, and therefore provide material rights. As such, the option rights at the time of an IND filing for each Option Target represent three separate performance obligations (one for each Option Target) as of the amendment of the arrangement. The license to each HMT inhibitor targeting each respective Option Target, the Company’s research and development obligations through the completion of a Phase 1 clinical trial for each Option Target, and the option to maintain the license beyond the end of Phase 1 clinical development for each Option Target are all subject to Celgene’s exercise of the option rights at the time of an IND filing and, therefore, are not considered performance obligations as of the amendment. Under the agreement, the Company determined that the total transaction price was $103.0 million as of the amendment of the arrangement, comprised the following: • $68.0 million total upfront payment received under the original agreement, as described above; • $25.0 million clinical development milestone payment for DOT1L; and • $10.0 million upfront payment under the amended and restated agreement. The option exercise fees of $75.0 million in the aggregate, for the options at the time of IND and completion of Phase 1, that may be received are excluded from the transaction price until each customer option is exercised. The future potential milestone payments were excluded from the transaction price, as all milestone amounts were fully constrained. The Company reevaluates 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 transaction price was allocated to the performance obligations based on the estimated stand-alone selling prices at the time of the amendment. For the DOT1L performance obligation that includes the license and pre-IND research services, the stand-alone selling price was determined considering the stage and status of the program and the technology involved and the level of development expected, as well as the expected cost and margin for the research services. For the post-IND research and development services for DOT1L and the pre-IND research services for each Option Target, the stand-alone selling price was determined considering the expected cost and a reasonable margin for the respective services. The material rights from the option rights at the time of an IND filing for each Option Target were valued based on the estimated discount at which the option is priced and the Company’s estimated probability of the options’ exercise as of the time of the amendment. The Company believes that a change in the assumptions used to determine its stand-alone selling price for the performance obligations most likely would not have a significant effect on the allocation of consideration received (or receivable) to the performance obligations that were not satisfied as of the adoption of ASC 606. The Company allocated the following amounts of the total transaction price to the performance obligations as of the amendment date: • $65.1 million, including the $25.0 million clinical development milestone payment for DOT1L, to the two DOT1L performance obligations, which were satisfied prior to the ASC 606 adoption date; • $34.1 million to the three Pre-IND research services performance obligations related to the Option Targets, which were substantially satisfied as of the ASC 606 adoption date; and • $3.8 million to the three material rights related to Celgene’s option rights at the time of an IND filing for each Option Target, which will not be satisfied until the option is exercised or one of the parties opts out of the arrangement. 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, 2018, 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, 2018, all of which is included in noncurrent liabilities. 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 as collaboration revenue a $20.0 million upfront payment, a $3.0 million payment upon the execution of the March 2014 agreement amendment, $6.0 million in fixed research funding, $9.0 million for research and development services and $51.0 million in preclinical research and development milestone payments. The preclinical and research and development milestone payments total includes a $10.0 million milestone payment earned in May 2017 related to the second target in the collaboration, upon GSK’s initiation of good laboratory practices toxicology studies, as well as a $6.0 million clinical milestone following GSK’s initiation of patient dosing in a Phase 1 clinical trial of a PRMT5 inhibitor that the Company discovered and licensed to GSK. In 2018 we recognized a $12.0 million milestone earned in 2018 relating to the first dosing of a patient in a Phase 2 clinical trial of GSK3326595, a PRMT5 inhibitor discovered by us and licensed to GSK under the collaboration agreement, as well as a $8.0 million milestone payment earned in the fourth quarter of 2018 relating to the initiation of a patient dosing in a Phase 1 clinical trial of GSK3368715, a PRMT1 inhibitor discovered by us and licensed to GSK under the collaboration agreement. As of December 31, 2018, 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, 2018, 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 and $10.0 million in milestone revenue in the year ended December 31, 2017. The Company did not have any deferred revenue related to this agreement as of December 31, 2018 or December 31, 2017 and any future revenues will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets. 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, under which the Company reacquired worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat. Under the amended and restated 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 waived the right of first negotiation for the rest of Asia. Under the original agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the amended and restated 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 Roche Molecular companion diagnostic agreement, 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 amended and restated agreement with Eisai 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 has 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 MAA, 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 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. LYSA In May 2016, the Company entered into a collaboration agreement with the Lymphoma Academic Research Organisation, or LYSARC, for the first planned combination trial of tazemetostat. LYSARC is the operational arm of the Lymphoma Study Association, or LYSA, a premier cooperative group in France dedicated to clinical and translational research for lymphoma. This Phase 1b/2 study is evaluating tazemetostat in combination with R-CHOP, the standard of care first line combination treatment for diffuse large B-cell lymphoma, or DLBCL, as a first line treatment in elderly, high-risk patients with DLBCL and is being sponsored by LYSARC. LYSA is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. Genentech In June 2016, the Company entered into a collaboration agreement with Genentech Inc. (“Genentech”), a member of the Roche Group, to conduct a Phase 1b clinical trial to investigate the anti-cancer effects of the Company’s EZH2 inhibitor, tazemetostat, and Genentech’s anti-PD-L1 cancer immunotherapy, atezolizumab, when used in combination. The trial is evaluating this combination regimen for the treatment of patients with relapsed or refractory DLBCL. Under the agreement, each company is supplying its respective anti-cancer agent to support the trial and sharing equally in the trial costs. Genentech is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. In June 2017, the Company announced an expansion of the clinical collaboration with Genentech to investigate the combination of tazemetostat with atezolizumab in a Phase 1b/2 clinical trial for the treatment of patients with relapsed or refractory metastatic non-small cell lung cancer, or NSCLC. The trial will be part of MORPHEUS, Genentech’s open-label, multi-center, randomized umbrella trial evaluating the efficacy and safety of multiple immunotherapy-based treatment combinations for metastatic NSCLC. Under the agreement, each company is supplying its respective anti-cancer agent to support the trial and sharing equally in the trial costs. Genentech is managing the study operations for the trial, and the Company is recognizing its share of the related expenses as those costs are incurred over the duration of the trial. 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 is 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. As of December 31, 2018, the Company is responsible for the remaining development costs of $8.4 million due under the agreement. We expect the remaining development costs under the amended agreement to be incurred and paid through 2020. 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 will receive $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 has 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. 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. The next potential milestone payment that the Company might be entitled to receive under this agreement is a $5.5 million milestone, for each Target Project, for the SoLO Approval for a compound, as defined in 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 is $20.0 million, comprised of the following: • $15.0 million total upfront payment received under the agreement; and • $5.0 million research funding payment to be received in 2019 The future potential milestone payments are excluded from the transaction price, 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 is allocated to the first undisclosed target license and associated research services and $10.0 million is allocated to the second undisclosed target license and associated research services and will be recognized through 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, 2018, the Company has recognized $1.7 million in total collaboration revenue since the inception of this collaboration. As of December 31, 2018, the Company had deferred revenue of $13.3 million related to this agreement. 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.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
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 Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2018 2017 2016 (In thousands) Research and development $ 4,083 $ 5,613 $ 5,352 General and administrative 7,921 5,818 5,216 Total $ 12,004 $ 11,431 $ 10,568 Stock Options The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. Key weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.6 % 1.8 % 1.2 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 71.5 % 74.2 % 78.5 % Expected dividend yield 0.0 % 0.0 % 0.0 % There were no stock option awards granted to non-employees in the years ended December 31, 2018, 2017 or 2016. 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, 2018: 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, 2017 4,576 $ 14.57 Granted 2,537 14.66 Exercised (215 ) 8.76 Forfeited or expired (1,745 ) 15.70 Outstanding at December 31, 2018 5,153 $ 14.48 7.7 $ 513 Exercisable at December 31, 2018 2,126 $ 15.33 6.1 $ 513 During the years ended December 31, 2018, 2017 and 2016, the Company granted stock options to purchase an aggregate of 2,537,277 shares, 2,331,500 shares, and 2,212,668 shares, respectively, at weighted average grant date fair values per option share of $9.49, $8.85, and $6.42, respectively. The total grant date fair value of options that vested during the years ended December 31, 2018, 2017 and 2016 was $12.1 million, $12.0 million, and $12.0 million, respectively. The aggregate intrinsic value of stock options exercised was $1.5 million in 2018, $4.1 million in 2017 and $7.1 million in 2016. As of December 31, 2018, there was $23.4 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 2.7 years. Restricted Stock Units As of December 31, 2018, there were no restricted stock units outstanding. In February 2016, the Company granted 80,732 restricted stock units with a grant date fair value of $9.29 per unit, in accordance with a former chief financial officer’s employment agreement. Of the total restricted stock unit awards granted, 73,779 shares vested through the termination of the officer’s employment in August 2017. As of December 31, 2018, there was no unrecognized compensation cost related to restricted stock units, which were cancelled upon termination for the remaining unvested awards. The intrinsic value of restricted stock that vested during the years ended December 31, 2018, 2017 and 2016 was $0.0 million, $0.3 million and $0.5 million, respectively. 401(k) Savings Plan The Company has a defined contribution 401(k) savings plan (the “401(k) Plan”). The 401(k) Plan covers substantially all 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 $0.5 million, $0.5 million and $0.4 million in the years ended December 31, 2018, 2017 and 2016, respectively.

Loss per Share

Loss per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
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, 2018 2017 2016 (In thousands except per share data) Net loss $ (123,630 ) $ (134,309 ) $ (110,212 ) Weighted average shares outstanding 71,864 61,471 57,126 Basic and diluted loss per share allocable to common stockholders $ (1.72 ) $ (2.18 ) $ (1.93 ) 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 antidilutive: Year Ended December 31, 2018 2017 2016 (In thousands) Stock options 5,153 4,576 4,059 Unvested restricted stock — — 64 Shares issuable under employee stock purchase plan 28 23 46 5,181 4,599 4,169

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 4.6% of the Company’s outstanding common stock as of December 31, 2018. Refer to Note 9, Collaborations Under the Celgene collaboration agreement, the Company recognized $1.9 million and $1.1 million in collaboration revenue in the years ended December 31, 2017 and 2016, respectively. The Company recognized no revenue under the Celgene collaboration agreement for the year ended December 31, 2018. As of December 31, 2018, and 2017 the Company had $3.8 million and $28.8 million in deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the years ended December 31, 2018, 2017 and 2016, the Company recorded $0.0 million, $0.0 million and $0.1 million, respectively, in global development co-funding from Celgene. As of December 31, 2018 and 2017, the Company had no accounts receivable for either period, respectively, related to this collaboration arrangement.

Unaudited Quarterly Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Results	The results of operations on a quarterly basis for the years ended December 31, 2018 and 2017 are set forth below: 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 benefit — — — (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 Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (In thousands, except per share data) Collaboration revenue $ — $ 10,000 $ — $ — Operating expenses: Research and development 24,695 27,292 28,741 28,933 General and administrative 8,269 11,170 9,311 8,431 Total operating expenses 32,964 38,462 38,052 37,364 Operating loss (32,964 ) (28,462 ) (38,052 ) (37,364 ) Other income, net 442 438 455 862 Income tax benefit — — — 336 Net loss $ (32,522 ) $ (28,024 ) $ (37,597 ) $ (36,166 ) Loss per share allocable to common stockholders: Basic $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Diluted $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Weighted average shares outstanding: Basic 58,219 58,377 59,899 69,287 Diluted 58,219 58,377 59,899 69,287

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation 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 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 within the consolidated balance sheets. 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, 2018 was $139.2 million, which consisted of 19 commercial paper securities and 29 corporate notes securities. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2017 was $42.8 million, which consisted of 4 commercial paper securities, 14 corporate notes securities, and 1 U.S. government agency security. 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 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, 2018 and 2017. 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 The following table summarizes the available for sale securities held at December 31, 2017 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 16,964 $ — $ (6 ) $ 16,958 Corporate notes 31,610 — (43 ) 31,567 U.S. government agency securities and U.S. Treasuries 1,250 — — 1,250 Total $ 49,824 $ — $ (49 ) $ 49,775 Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. All marketable securities held at December 31, 2018 and 2017 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, 2018, 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, 2018 and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. The aggregate fair value of available-for-sale securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2018 was $139.2 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of December 31, 2018 was less than $0.1 million. The Company determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2018. The weighted-average maturity of the Company’s portfolio was approximately two months at December 31, 2018.
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, 2018 and 2017 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of December 31, 2018 and December 31, 2017, the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 207,251 $ 207,251 $ — $ — Marketable securities: Commercial paper 16,958 — 16,958 — Corporate notes 31,567 — 31,567 — U.S. government agency securities and treasuries 1,250 — 1,250 — Total $ 257,026 $ 207,251 $ 49,775 $ — 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. 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.
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 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, 2018, 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, 2018 or 2017, 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 5 - 10 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 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 2018, 2017 or 2016.
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 On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”). This legislation makes broad and complex changes to the U.S. tax code, including, but not limited to, (i) reducing the U.S. federal statutory tax rate from 35% to 21%; (ii) eliminating the corporate alternative minimum tax (AMT) and changing how existing AMT credits can be realized; (iii) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017, (iv) modifying the officer’s compensation limitation, (v) changing rules related to the deductibility of entertainment expenses beginning in 2018, and (vi) changing rules related to the deductibility of qualified transportation benefits beginning in 2018. The Company recognizes the effects of changes in tax law, including the TCJA, in the period the law is enacted. Accordingly, the effects of the TCJA have been recognized in the financial statements as applicable for the years ended December 31, 2017 and December 31, 2018. In December 2017, the SEC staff issued Staff Accounting Bulletin, or SAB, No. 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of TCJA. As of December 31, 2017, the effects of the TCJA were recorded on a provisional basis. As of December 31, 2018, the Company finalized its accounting for the TCJA and no measurement adjustments were recorded.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification, or 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 partner related to the licensed targets. Payments to the Company under these agreements may include non-refundable license fees, customer option exercise fees, payments for research activities, reimbursement of certain costs, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. The Company first evaluates license and/or collaboration arrangements to determine whether the arrangement (or part of the arrangement) represents a collaborative arrangement pursuant to ASC Topic 808, Collaborative Arrangements, , which represent a collaborative relationship and not a customer relationship, Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Amounts recognized as revenue, but not yet received or invoiced are generally recognized as contract assets. Exclusive Licenses – If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, which generally include research and development services, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a license is distinct from the other promises, the Company considers relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from the license for its intended purpose without the receipt of the remaining promises, whether the value of the license is dependent on the unsatisfied promises, whether there are other vendors that could provide the remaining promises, and whether it is separately identifiable from the remaining promises. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Research and Development Services – The promises under the Company’s collaboration and license agreements generally include research and development services to be performed by the Company on behalf of the collaboration partner. For performance obligations that include research and development services, the Company generally recognizes revenue allocated to such performance obligations based on an appropriate measure of progress. The Company utilizes judgment to determine the appropriate method of measuring progress for purposes of recognizing revenue, which is generally an input measure such as costs incurred. The Company evaluates the measure of progress each reporting period as described under above. Reimbursements from the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options – The Company’s arrangements may provide a collaborator with the right to select a target for licensing either at the inception of the arrangement or within an initial pre-defined selection period, which may, in certain cases, include the right of the collaborator to extend the selection period. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement as an upfront fee or payment, (ii) upon the exercise of an option to acquire a license or (iii) upon extending the selection period as an extension fee or payment. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the inception of the arrangement. The Company allocates the transaction price to material rights based on the relative stand-alone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised or expires. Milestone Payments – At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. If a milestone or other variable consideration relates specifically to the Company’s efforts to satisfy a single performance obligation or to a specific outcome from satisfying the performance obligation, the Company generally allocates the milestone amount entirely to that performance obligation once it is probable that a significant revenue reversal would not occur. Royalties – For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 9, 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. The Company has entered into collaboration and license agreements to discover, develop, manufacture and commercialize compounds directed to specific HMT targets. The terms of these agreements typically contain multiple deliverables, which may include: (i) licenses, or options to obtain licenses, to compounds directed to specific HMT targets (referred to as “exclusive licenses”) and (ii) research and development activities to be performed on behalf of the collaboration partner related to the licensed HMT targets. Payments to the Company under these agreements may include non-refundable license fees, option fees, exercise fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. 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. Our 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, we 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. We have determined that some of our exclusive licenses lack standalone value apart from the related research and development services, and in those circumstances we 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 our 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 our central operations, when we are principally responsible for the performance of these services under the agreements, we recognized revenue on a gross basis for research and development services as those services were performed. • Option Arrangements. Our 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 us 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, we are 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, we did not consider the licenses to be deliverables at the inception of the arrangement. For arrangements where the option to secure licenses is not considered substantive, we considered 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 . Our 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. We 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 and regulatory milestones that were expected to be achieved as a result of our efforts during the period of our performance obligations under the collaboration 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 was initially deferred and recognized over the remaining term of our performance obligations. Milestones that were not considered substantive because we did not contribute effort to their achievement are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met, as there are no undelivered elements remaining and no continuing performance obligations on our 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 third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its product candidates incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history for related activities and the expected duration of the third-party 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. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to vendors will exceed the level of services provided and result in a prepayment of 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 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. Refer to Note 10, Employee Benefit Plans
Earnings (Loss) per Share	Earnings (Loss) per Share The Company computes basic earnings (loss) per share by dividing income (loss) allocable to common stockholders by the weighted average number of shares of common stock outstanding. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s restricted stock participates 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 that are outstanding during the period, except where such non-participating securities would be anti-dilutive. Refer to Note 11, 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 February 2016, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2016-02, Leases (Topic 842) Currently, the Company is gathering information, reviewing its portfolio of existing leases, and continuing to evaluate the potential changes to the Company’s future financial reporting and disclosures that may result from adopting this ASU. The Company plans to elect the practical expedient which will allow it to not apply the amended lease accounting guidance to comparative periods that will be presented. The Company expects that all of its lease commitments will be subject to the new standard with the cumulative effect of adoption recognized to retained earnings on January 1, 2019. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808)
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Revenue Recognition In May 2014, the FASB, issued ASU, 2014-09, Revenue From Contracts With Customers . ASU 2014-09 amends Accounting Standards Codification, or ASC, 605, Revenue Recognition (“ASC 605”), by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. In addition, the FASB issued ASUs 2016-10 and 2016-12, which provide clarifying amendments to ASU 2014-09. ASU 2014-09 and its related amendments were effective for the Company for interim and annual periods in 2018. The new standards are codified under ASC 606. As a result of adopting ASC 606 on January 1, 2018, the Company recorded a cumulative-effect credit to opening accumulated deficit of $25.0 million as of January 1, 2018 and a corresponding decrease to deferred revenue, net of current portion. The cumulative-effect change relates principally to the Company’s treatment of option rights under its agreement with Celgene Corporation, or “Celgene”, and the identification of more performance obligations under ASC 606 in comparison with identified units of accounting under ASC 605. The adoption did not impact the previous accounting for the Company’s agreements with Glaxo Group Limited, or “GSK” and Eisai Co. Ltd., or Eisai. Pursuant to ASC 605, the Company had deemed Celgene’s options to license the three small molecule HMT inhibitors targeting three predefined targets, or the Option Targets, as non-substantive and therefore included the services that it would be required to perform upon option exercise as deliverables. ASC 606 provides that only options that are deemed to be material rights are a performance obligation and that any goods or services required upon exercise of the option be excluded from the evaluation of performance obligations until the option is exercised. As a result of this change to the guidance, (1) the pre-IND research services performed by the Company for each of the three Option Targets were deemed to be distinct performance obligations whereas each had previously been combined into one unit of accounting with the respective license that is subject to the exercise of the option and (2) a lesser amount of transaction price was allocated to the options. For further discussion of the change and the adoption of this standard, see Note 9, Collaborations For the twelve months ended December 31, 2018, we recognized $21.7 million in collaboration revenue in accordance with ASC 606, which was materially consistent with what we would have recorded under ASC 605. Deferred revenue as of December 31, 2018 was $17.1 million under ASC 606, as compared to a balance of $42.1 million, which would have resulted under ASC 605. Cash As of January 1, 2018, the Company adopted ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments 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, 2018 2017 2016 (In thousands) Cash and cash equivalents $ 86,671 $ 226,664 $ 77,895 Restricted cash, as part of other assets 462 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 87,133 $ 227,126 $ 78,357 The $0.5 million relates to a letter of credit as a security deposit for the office and laboratory lease at Technology Square in Cambridge, Massachusetts. The Company has recorded cash held to secure this letter of credit as restricted cash in restricted cash and other assets on the consolidated balance sheet. Share-Based Payment As of January 1, 2018, the Company adopted ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Available-for-Sale Securities Held	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 The following table summarizes the available for sale securities held at December 31, 2017 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 16,964 $ — $ (6 ) $ 16,958 Corporate notes 31,610 — (43 ) 31,567 U.S. government agency securities and U.S. Treasuries 1,250 — — 1,250 Total $ 49,824 $ — $ (49 ) $ 49,775
Summary of Company's Financial Assets Recognized at Fair Value	As of December 31, 2018 and December 31, 2017, the Company’s financial assets recognized at fair value consisted of the following: 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 $ — Fair Value as of December 31, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 207,251 $ 207,251 $ — $ — Marketable securities: Commercial paper 16,958 — 16,958 — Corporate notes 31,567 — 31,567 — U.S. government agency securities and treasuries 1,250 — 1,250 — Total $ 257,026 $ 207,251 $ 49,775 $ —
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 5 - 10 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 years or term of respective lease, if shorter
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, 2018 2017 2016 (In thousands) Cash and cash equivalents $ 86,671 $ 226,664 $ 77,895 Restricted cash, as part of other assets 462 462 462 Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows $ 87,133 $ 227,126 $ 78,357

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net consists of the following: December 31, 2018 2017 (In thousands) Laboratory equipment $ 4,132 $ 4,138 Computer and office equipment, furniture (1) 5,040 4,807 Leasehold improvements 414 354 Construction in progress 271 71 Property and equipment 9,857 9,370 Less: accumulated depreciation and amortization (7,800 ) (6,843 ) Property and equipment, net $ 2,057 $ 2,527 (1) In 2015, the Company acquired $1.7 million in computer hardware and equipment, pursuant to a capital lease, the term of which expires in February 2018. In April 2018, the Company acquired $0.1 million of equipment pursuant to a capital lease, the term of which expires in April 2023. Accumulated depreciation related to these assets totaled $0.0 million and $1.6 million as of December 31, 2018 and 2017, respectively.

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: December 31, 2018 2017 (In thousands) Prepaid clinical and manufacturing costs $ 6,295 $ 5,724 Interest receivable on available for sale securities 679 249 Other prepaid expenses and other receivables 5,190 3,010 Total prepaid expenses and other current assets $ 12,164 $ 8,983

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: December 31, 2018 2017 (In thousands) Employee compensation and benefits $ 5,509 $ 4,628 Research and development expenses 11,272 11,658 Professional services and other 2,919 1,263 Accrued expenses $ 19,700 $ 17,549

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018, 2017, and 2016 is as follows: 2018 2017 2016 (In thousands) Current $ (127 ) $ 32 $ — Deferred 184 (368 ) — Total 57 (336 ) — Income tax provision (benefit) $ 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, 2018 2017 2016 Federal statutory income tax rate 21.0 % 34.0 % 34.0 % State income taxes 5.7 4.8 4.5 Research and development and other tax credits 2.4 4.2 3.3 Permanent items (0.7 ) (2.0 ) (1.9 ) Change in valuation allowance (28.3 ) 5.7 (40.1 ) Return-to-provision adjustments (0.1 ) (0.7 ) — Change in deferred taxes — — 0.2 Rate Change — (45.7 ) — Effective income tax rate 0.0 % 0.3 % 0.0 %
Deferred Tax Assets (Liabilities) Included in Other Assets in Consolidated Balance Sheet	The Company’s deferred tax assets (liabilities) included in other assets in the consolidated balance sheets consist of the following: December 31, 2018 2017 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 147,586 $ 116,869 Research and development and other credit carryforwards 26,731 24,257 Capitalized start-up costs 1,031 1,175 Deferred revenue 1,021 7,766 Accruals and allowances 1,522 1,250 Eisai license payment 8,225 8,985 Other 5,172 4,738 Gross deferred tax assets 191,288 165,040 Deferred tax asset valuation allowance (191,070 ) (164,607 ) Total deferred tax assets 218 433 Deferred tax liabilities: Depreciation and other (34 ) (65 ) Total deferred tax liabilities (34 ) (65 ) Net deferred tax asset (liability) $ 184 $ 368
Summary of Unrecognized Tax Benefits	The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2018 2017 (In thousands) Unrecognized tax benefits - as of beginning of year $ 5,223 $ 4,206 Gross increases - current period tax positions 520 1,017 Unrecognized tax benefits - as of end of year $ 5,743 $ 5,223

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Contractual Commitments	The Company’s contractual commitments under these leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2018 are 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 $ —

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 (In thousands) Research and development $ 4,083 $ 5,613 $ 5,352 General and administrative 7,921 5,818 5,216 Total $ 12,004 $ 11,431 $ 10,568
Weighted Average Assumptions Used in Applying Pricing Model	Key weighted average assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.6 % 1.8 % 1.2 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 71.5 % 74.2 % 78.5 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the year ended December 31, 2018: 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, 2017 4,576 $ 14.57 Granted 2,537 14.66 Exercised (215 ) 8.76 Forfeited or expired (1,745 ) 15.70 Outstanding at December 31, 2018 5,153 $ 14.48 7.7 $ 513 Exercisable at December 31, 2018 2,126 $ 15.33 6.1 $ 513

Loss per Share (Tables)

Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 (In thousands except per share data) Net loss $ (123,630 ) $ (134,309 ) $ (110,212 ) Weighted average shares outstanding 71,864 61,471 57,126 Basic and diluted loss per share allocable to common stockholders $ (1.72 ) $ (2.18 ) $ (1.93 )
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 antidilutive: Year Ended December 31, 2018 2017 2016 (In thousands) Stock options 5,153 4,576 4,059 Unvested restricted stock — — 64 Shares issuable under employee stock purchase plan 28 23 46 5,181 4,599 4,169

Unaudited Quarterly Results (Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 are set forth below: 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 benefit — — — (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 Quarter Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (In thousands, except per share data) Collaboration revenue $ — $ 10,000 $ — $ — Operating expenses: Research and development 24,695 27,292 28,741 28,933 General and administrative 8,269 11,170 9,311 8,431 Total operating expenses 32,964 38,462 38,052 37,364 Operating loss (32,964 ) (28,462 ) (38,052 ) (37,364 ) Other income, net 442 438 455 862 Income tax benefit — — — 336 Net loss $ (32,522 ) $ (28,024 ) $ (37,597 ) $ (36,166 ) Loss per share allocable to common stockholders: Basic $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Diluted $ (0.56 ) $ (0.48 ) $ (0.63 ) $ (0.52 ) Weighted average shares outstanding: Basic 58,219 58,377 59,899 69,287 Diluted 58,219 58,377 59,899 69,287

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 81,602	$ 152,534	$ 129,955
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 240,300
Accumulated deficit	(586,724)	$ (488,097)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	988,200
Non-equity funding through collaboration agreement	232,800
IPO [Member]
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 679,400

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	Jul. 08, 2015OptionTarget	Dec. 31, 2018USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2018USD ($)SecuritySegmentOptionTargetAccountingUnitshares	Dec. 31, 2017USD ($)Security	Dec. 31, 2016USD ($)	Dec. 31, 2018USD ($)	Jan. 01, 2018USD ($)
Accounting Policies [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		$ 139,200,000			$ 139,200,000	$ 42,800,000		$ 139,200,000
Investments with an other-than-temporary impairment					0	0
Realized gains (losses) recognized on sale or maturity of marketable equity securities					0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss		100,000			$ 100,000			100,000
Weighted average maturity period					2 months
Allowance for doubtful accounts		0			$ 0	0		0
Impairment of long-lived assets					$ 0	$ 0	$ 0
Largest amount of tax benefit					50.00%
Federal statutory income tax rate					21.00%	34.00%	34.00%
TCJA measurement adjustments					$ 0
Reportable business segment | Segment					1
Collaboration revenue		9,700,000	$ 12,000,000	$ 10,000,000	$ 21,700,000	$ 10,000,000	$ 8,007,000
Deferred revenue						$ 42,100,000
Security deposit		500,000			500,000			500,000
Letter of Credit [Member]
Accounting Policies [Line Items]
Security deposit		500,000			$ 500,000			500,000
Celgene [Member]
Accounting Policies [Line Items]
Number of option targets | OptionTarget	3				3
Number of accounting units | AccountingUnit					1
Collaboration revenue								99,200,000
Deferred revenue		3,800,000			$ 3,800,000			3,800,000
ASC 606 [Member]
Accounting Policies [Line Items]
Cumulative-effect credit to opening accumulated deficit									$ 25,000,000
Collaboration revenue					21,700,000
Deferred revenue		$ 17,100,000			$ 17,100,000			$ 17,100,000
ASU 2017-09 [Member]
Accounting Policies [Line Items]
Stock-based compensation expense awards modified | shares					0
Maximum [Member]
Accounting Policies [Line Items]
Federal statutory income tax rate						35.00%
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					19	4
Corporate Notes [Member]
Accounting Policies [Line Items]
Number of securities in unrealized loss position | Security					29	14

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 153,685	$ 49,824
Available-For-Sale Securities Held, Unrealized Losses	(52)	(49)
Available-For-Sale Securities Held, Fair Value	153,633	49,775
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	73,110	16,964
Available-For-Sale Securities Held, Unrealized Losses	(22)	(6)
Available-For-Sale Securities Held, Fair Value	73,088	16,958
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	80,575	31,610
Available-For-Sale Securities Held, Unrealized Losses	(30)	(43)
Available-For-Sale Securities Held, Fair Value	$ 80,545	31,567
U.S. Government Agency Securities and Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost		1,250
Available-For-Sale Securities Held, Fair Value		$ 1,250

Summary of Significant Accoun_6

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, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	$ 79,225	$ 207,251
Total	232,858	257,026
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	73,088	16,958
Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	80,545	31,567
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities		1,250
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	50,785	207,251
Total	50,785	207,251
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash equivalents	28,440
Total	182,073	49,775
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	73,088	16,958
Level 2 [Member] | Corporate Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities	$ 80,545	31,567
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Marketable securities		$ 1,250

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	12 Months Ended
	Dec. 31, 2018
Laboratory Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	5 years
Laboratory Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Office Furniture and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Office Furniture and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 86,671	$ 226,664	$ 77,895
Restricted cash, as part of other assets	462	462	462
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 87,133	$ 227,126	$ 78,357

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment	$ 9,857	$ 9,370
Less: accumulated depreciation and amortization	(7,800)	(6,843)
Property and equipment, net	2,057	2,527
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,132	4,138
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	5,040	4,807
Less: accumulated depreciation and amortization	0	(1,600)
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	414	354
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 271	$ 71

Property and Equipment, Net -_2

Property and Equipment, Net - Schedule of Property and Equipment, Net (Parenthetical) (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2015	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Accumulated depreciation			$ 7,800	$ 6,843
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Computer hardware and equipment acquired	$ 100	$ 1,700
Expiration of capital lease term	Apr. 30, 2023	Feb. 28, 2018
Accumulated depreciation			$ 0	$ 1,600

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,052	$ 1,639	[1]	$ 1,589	[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, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid clinical and manufacturing costs	$ 6,295	$ 5,724
Interest receivable on available for sale securities	679	249
Other prepaid expenses and other receivables	5,190	3,010
Total prepaid expenses and other current assets	$ 12,164	$ 8,983

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Employee compensation and benefits	$ 5,509	$ 4,628
Research and development expenses	11,272	11,658
Professional services and other	2,919	1,263
Accrued expenses	$ 19,700	$ 17,549

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, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Current			$ (127,000)	$ 32,000
Deferred			184,000	(368,000)	[1]
Total	$ 57,000	$ (336,000)	57,000	(336,000)
Income tax provision (benefit)	$ 57,000	$ (336,000)	$ 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	34.00%	34.00%
State income taxes	5.70%	4.80%	4.50%
Research and development and other tax credits	2.40%	4.20%	3.30%
Permanent items	(0.70%)	(2.00%)	(1.90%)
Change in valuation allowance	(28.30%)	5.70%	(40.10%)
Return-to-provision adjustments	(0.10%)	(0.70%)
Change in deferred taxes			0.20%
Rate Change		(45.70%)
Effective income tax rate	0.00%	0.30%	0.00%

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets (Liabilities) Included in Other Assets in Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 147,586	$ 116,869
Research and development and other credit carryforwards	26,731	24,257
Capitalized start-up costs	1,031	1,175
Deferred revenue	1,021	7,766
Accruals and allowances	1,522	1,250
Eisai license payment	8,225	8,985
Other	5,172	4,738
Gross deferred tax assets	191,288	165,040
Deferred tax asset valuation allowance	(191,070)	(164,607)
Total deferred tax assets	218	433
Deferred tax liabilities:
Depreciation and other	(34)	(65)
Total deferred tax liabilities	(34)	(65)
Net deferred tax asset (liability)	$ 184	$ 368

Income Taxes (Tax Cuts and Jobs

Income Taxes (Tax Cuts and Jobs Act) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred income taxes	$ 184,000	$ (368,000)	[1]
Income tax benefit	$ 0	$ 61,600,000
[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)	12 Months Ended
	Dec. 31, 2018USD ($)
United States, Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 542,600,000
Operating loss carryforwards, expire at various dates from 2029 through 2037	$ 428,500,000
Operating loss carryforwards, expiration beginning year	2,029
Operating loss carryforwards, expire indefinitely	$ 114,100
Operating loss carryforwards, expiration ending year	2,037
United States, State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 535,400,000
Operating loss carryforwards, expiration beginning year	2,030

Income Taxes (Research and Deve

Income Taxes (Research and Development Tax Credit) - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Tax Credit Carryforward [Line Items]
Research and development tax credit carryforward expiration beginning year	2,028
Research and development tax credit carryforward expiration ending year	2,038
Federal orphan drug tax credit carryforwards expiration beginning year	2,033
Expiry period of federal orphan drug tax credit carryforwards	2,038
Federal orphan drug tax credit carryforwards	$ 15,200,000
Federal alternative minimum tax credit	200,000
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	9,100,000
Research Tax Credit Carryforward [Member] | United States, State [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	$ 2,900,000

Income Taxes - Summary of Unrec

Income Taxes - Summary of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Unrecognized tax benefits - as of beginning of year	$ 5,223	$ 4,206
Gross increases - current period tax positions	520	1,017
Unrecognized tax benefits - as of end of year	$ 5,743	$ 5,223

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018
Other Commitments [Line Items]
Monthly base rent	$ 200,000
Increase in base rent	33,000
Annual increase of base rent	9,000
Letter of credit as a security deposit	500,000
Rental expense	3,500,000	$ 3,300,000	$ 2,800,000
Roche [Member]
Other Commitments [Line Items]
Payment obligation incurred and paid	2,000,000	$ 1,500,000
Collaborative Arrangement [Member] | Roche [Member]
Other Commitments [Line Items]
Remaining development costs	8,400,000			$ 10,400,000
Reimbursements receivable of development costs	$ 900,000
Initial Term [Member]
Other Commitments [Line Items]
Lease expiration date	May 31, 2018
Extended Term [Member]
Other Commitments [Line Items]
Lease expiration date	Nov. 30, 2022

Commitments and Contingencies_2

Commitments and Contingencies - Contractual Commitments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [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) $ in Thousands	Mar. 10, 2017shares	Oct. 31, 2018shares	Sep. 30, 2017shares	Apr. 30, 2016	Jan. 31, 2016shares	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)Voteshares	Dec. 31, 2017USD ($)shares		Dec. 31, 2016USD ($)shares
Equity [Line Items]
Number of votes for each share of common stock | Vote							1
Common Stock Sold through ATM Facility | shares		9,583,334	10,557,000		15,333,334
Proceeds from Issuance of Common Stock							$ 81,938	$ 152,922	[1]	$ 130,438	[1]
Common Stock Issuance Cost Incurred							$ 260	$ 388	[1]	$ 483	[1]
Common stock reserved for future issuance | shares							11,090,646
At The Market Facility [Member]
Equity [Line Items]
Common Stock Sold through ATM Facility | shares	155,834							132,253		23,581
Commission rate for sales agent				3.00%
Proceeds from Issuance of Common Stock								$ 1,600		$ 300
Common Stock Issuance Cost Incurred						$ 100
Equity offering sales agreement termination date							Mar. 10, 2017
[1]	Revised as a result of the adoption of ASU 2016-18

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015USD ($)OptionTarget	Jan. 01, 2011OptionTarget	Nov. 30, 2018USD ($)Installment	May 31, 2017USD ($)	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017	Jun. 30, 2017USD ($)	Mar. 31, 2017	Dec. 31, 2018USD ($)OptionTargetSeparatePerformanceObligationPerformanceObligationMaterialRight	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue							$ 9,700,000		$ 12,000,000				$ 10,000,000		$ 21,700,000	$ 10,000,000	$ 8,007,000
Deferred revenue											$ 42,100,000					$ 42,100,000
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
DOT1L [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone payment	$ 35,000,000
DOT1L [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments	$ 100,000,000
DOT1l Performance Obligations [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price															$ 65,100,000
Number of performance obligations | PerformanceObligation															2
Pre-IND Research Services Performance Obligations [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price															$ 34,100,000
Number of performance obligations | PerformanceObligation															3
Material Rights [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price															$ 3,800,000
Number of material rights | MaterialRight															3
Roche [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Notice period in days															90 days
Collaborative Arrangement [Member] | Roche [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining unpaid milestone payments							$ 8,400,000			$ 10,400,000					$ 8,400,000				$ 8,400,000	$ 8,400,000
Reimbursements receivable of development costs							900,000								900,000				900,000	900,000
Celgene [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Company received upfront payment																			65,000,000
Proceeds from redeemable convertible preferred stock																			25,000,000
Upfront payment received															$ 68,000,000				68,000,000
Clinical development milestone achieved																			25,000,000
Global development co-funding																			7,000,000
Number of option targets | OptionTarget	3														3
Number of separate performance obligations | SeparatePerformanceObligation															3
Total transaction price															$ 103,000,000
Option Exercise Fee															75,000,000
Collaboration revenue																			99,200,000
Deferred revenue							3,800,000								3,800,000				3,800,000	3,800,000
Celgene [Member] | Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] | United States [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of option targets | OptionTarget	2
Celgene [Member] | Non-US [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of option targets | OptionTarget	1
Celgene [Member] | DOT1L [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone achieved															25,000,000
Clinical development milestone payment							35,000,000								35,000,000				35,000,000	35,000,000
Additional milestone payments							100,000,000								100,000,000				100,000,000	100,000,000
Celgene [Member] | Available Targets [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments							100,000,000								100,000,000				100,000,000	100,000,000
Additional payments							65,000,000								65,000,000				65,000,000	65,000,000
Celgene [Member] | Option Targets [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Company received upfront payment	$ 10,000,000														10,000,000
Celgene [Member] | Option Targets [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments	365,000,000
Development milestone and license payments associated with the Option Targets	75,000,000
Sales-based milestone payments	$ 170,000,000
GSK [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received						$ 3,000,000
Clinical development milestone achieved				$ 6,000,000			8,000,000								12,000,000
Clinical development milestone payment							50,000,000								50,000,000				50,000,000	50,000,000
Additional milestone payments							197,000,000								197,000,000				197,000,000	197,000,000
Number of option targets | OptionTarget		3
Sales-based milestone payments							128,000,000								128,000,000				128,000,000	128,000,000
Collaboration revenue															20,000,000	$ 10,000,000
Fixed research funding received																				6,000,000
Milestone payments received																				51,000,000
Research and development services																				9,000,000
Preclinical and research and development milestone payments received				$ 10,000,000
Additional payments received															$ 0
Cash and accounts receivable																				89,000,000
Type of Revenue [Extensible List]															epzm:CollaborationRevenueMember	epzm:CollaborationRevenueMember
Deferred revenue							0				$ 0				$ 0	$ 0			0	0
GSK [Member] | Collaboration Revenue [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received																		$ 20,000,000
Eisai [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment made					$ 40,000,000
Clinical development milestone payments obligation							20,000,000								20,000,000				20,000,000	20,000,000
Regulatory milestone payments obligation							50,000,000								50,000,000				50,000,000	50,000,000
Eisai [Member] | First Submission of NDA or MAA [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone payments obligation							10,000,000								10,000,000				10,000,000	10,000,000
Regulatory milestone payments obligation							25,000,000								25,000,000				25,000,000	25,000,000
Boehringer Ingelheim [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Company received upfront payment			$ 15,000,000
Upfront payment received															15,000,000
Total transaction price															20,000,000
Collaboration revenue															1,700,000
Deferred revenue							13,300,000								13,300,000				13,300,000	13,300,000
Research funding for costs to be incurred			$ 5,000,000				$ 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
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
Boehringer Ingelheim [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Clinical development milestone payment			80,500,000
Regulatory milestone payments			106,500,000
Sales-based milestone payments			$ 93,500,000

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2017	Feb. 29, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting period			4 years
Vesting terms			25.0% vesting upon the one-year anniversary of the grant and the remaining 75.0% vesting monthly over the following three years
Incentive and non-qualified stock options, expiration period			10 years
Non-employee stock option awards granted			0	0	0
Expected dividend yield			0.00%
Aggregate stock options granted to employees, including a non-employee			2,537,277	2,331,500	2,212,668
Weighted-average fair value of options granted			$ 9.49	$ 8.85	$ 6.42
Grant date fair value of options vested			$ 12,100,000	$ 12,000,000	$ 12,000,000
Aggregate intrinsic value of stock option exercised			1,500,000	4,100,000	7,100,000
Unrecognized compensation cost related to stock options			$ 23,400,000
Expected weighted average period for recognition of compensation cost			2 years 8 months 12 days
Percentage of employer's matching contribution						50.00%
Maximum percentage of employee contribution						3.00%
Contribution to 401 (k) plan			$ 500,000	500,000	400,000
Defined contribution plan name			401(k) savings plan
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding			0
Unrecognized compensation cost related to restricted stock units			$ 0
Restricted Stock Units [Member] | February 9, 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted		80,732
Weighted Average Grant Date Fair Value per Share, Granted		$ 9.29
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares vested	73,779
Intrinsic value of restricted stock vested during period			$ 0	$ 300,000	$ 500,000
Vesting Year One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting percentage			25.00%
Vesting Remaining Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting percentage			75.00%

Employee Benefit Plans - Schedu

Employee Benefit Plans - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 12,004	$ 11,431	[1]	$ 10,568	[1]
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	4,083	5,613		5,352
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 7,921	$ 5,818		$ 5,216
[1]	Revised as a result of the adoption of ASU 2016-18

Employee Benefit Plans - Weight

Employee Benefit Plans - Weighted Average Assumptions Used in Applying Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	2.60%	1.80%	1.20%
Expected life of options	6 years	6 years	6 years
Expected volatility of underlying stock	71.50%	74.20%	78.50%
Expected dividend yield	0.00%	0.00%	0.00%

Employee Benefit Plans - Summar

Employee Benefit Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding, Beginning balance	4,576,000
Number of Options, Granted	2,537,277	2,331,500	2,212,668
Number of Options, Exercised	(215,000)
Number of Options, Forfeited or expired	(1,745,000)
Number of Options, Outstanding, Ending balance	5,153,000	4,576,000
Number of Options, Exercisable	2,126,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 14.57
Weighted Average Exercise Price per Share, Granted	14.66
Weighted Average Exercise Price per Share, Exercised	8.76
Weighted Average Exercise Price per Share, Forfeited or expired	15.70
Weighted Average Exercise Price per Share, Outstanding, Ending balance	14.48	$ 14.57
Weighted Average Exercise Price per Share, Exercisable	$ 15.33
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	$ 513
Aggregate Intrinsic Value, Exercisable	$ 513

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss	$ (22,945)	$ (37,492)	$ (29,128)	$ (34,065)	$ (36,166)	$ (37,597)	$ (28,024)	$ (32,522)	$ (123,630)	$ (134,309)	$ (110,212)
Weighted average shares outstanding									71,864	61,471	57,126
Basic and diluted loss per share allocable to common stockholders									$ (1.72)	$ (2.18)	$ (1.93)

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	5,181	4,599	4,169
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	5,153	4,576	4,059
Unvested Restricted 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			64
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	28	23	46

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)

3 Months Ended

12 Months Ended

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2013

Related Party Transaction [Line Items]

Equity investments in common stock

79,175,000

69,302,000

79,175,000

69,302,000

Collaboration revenue related to agreement

$ 9,700,000

$ 12,000,000

$ 10,000,000

$ 21,700,000

$ 10,000,000

$ 8,007,000

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

Beneficial Owner [Member]

Related Party Transaction [Line Items]

Equity investments in common stock

3,674,640

Equity investments in outstanding common stock percentage

4.60%

4.60%

Collaboration revenue related to agreement

$ 0

$ 1,900,000

$ 1,100,000

Type of Revenue [Extensible List]

epzm:CollaborationRevenueMember

epzm:CollaborationRevenueMember

epzm:CollaborationRevenueMember

Deferred revenue

$ 3,800,000

$ 28,800,000

$ 3,800,000

$ 28,800,000

Global development co-funding from Celgene

0

0

$ 100,000

Accounts receivable related to collaboration arrangement

$ 0

$ 0

$ 0

$ 0

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 9,700		$ 12,000				$ 10,000		$ 21,700	$ 10,000	$ 8,007
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	$ 21,838	$ 27,027	$ 31,346	$ 25,622	$ 28,933	$ 28,741	$ 27,292	$ 24,695	$ 105,833	$ 109,661	$ 91,461
General and administrative	12,170	11,528	10,914	9,360	8,431	9,311	11,170	8,269	43,972	37,181	28,372
Total operating expenses	34,008	38,555	42,260	34,982	37,364	38,052	38,462	32,964	149,805	146,842	119,833
Operating loss	(24,308)	(38,555)	(30,260)	(34,982)	(37,364)	(38,052)	(28,462)	(32,964)	(128,105)	(136,842)	(111,826)
Other income, net	1,420	1,063	1,132	917	862	455	438	442	4,532	2,197	1,614
Income tax benefit	(57)				336				(57)	336
Net loss	$ (22,945)	$ (37,492)	$ (29,128)	$ (34,065)	$ (36,166)	$ (37,597)	$ (28,024)	$ (32,522)	$ (123,630)	$ (134,309)	$ (110,212)
Loss per share allocable to common stockholders:
Basic	$ (0.29)	$ (0.54)	$ (0.42)	$ (0.49)	$ (0.52)	$ (0.63)	$ (0.48)	$ (0.56)	$ (1.72)	$ (2.18)	$ (1.93)
Diluted	$ (0.29)	$ (0.54)	$ (0.42)	$ (0.49)	$ (0.52)	$ (0.63)	$ (0.48)	$ (0.56)	$ (1.72)	$ (2.18)	$ (1.93)
Weighted average shares outstanding:
Basic	78,962	69,539	69,490	69,386	69,287	59,899	58,377	58,219	71,864	61,471	57,126
Diluted	78,962	69,539	69,490	69,386	69,287	59,899	58,377	58,219	71,864	61,471	57,126