Epizyme (EPZM) 10-K 13 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Mar. 01, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
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	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		58,248,726
Entity Public Float			$ 449.4

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents	$ 77,895	$ 208,323
Marketable securities	164,297
Accounts receivable	23	262
Prepaid expenses and other current assets	6,457	4,478
Total current assets	248,672	213,063
Property and equipment, net	3,124	4,089
Restricted cash and other assets	645	751
Total Assets	252,441	217,903
Current Liabilities:
Accounts payable	4,994	4,653
Accrued expenses	16,007	11,335
Current portion of capital lease obligation	620	561
Current portion of deferred revenue		1,900
Total current liabilities	21,621	18,449
Capital lease obligation, net of current portion	110	730
Deferred revenue, net of current portion	28,809	28,809
Other long-term liabilities	201	383
Commitments and contingencies (Note 7)
Stockholders' Equity:
Common stock, $0.0001 par value; 125,000 shares authorized; 58,050 shares and 41,786 shares issued and outstanding, respectively	6	4
Additional paid-in capital	555,473	412,989
Accumulated other comprehensive loss	(106)
Accumulated deficit	(353,673)	(243,461)
Total stockholders' equity	201,700	169,532
Total Liabilities and Stockholders' Equity	$ 252,441	$ 217,903

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	58,050,000	41,786,000
Common stock, shares outstanding	58,050,000	41,786,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Collaboration revenue	$ 8,007	$ 2,560	$ 41,411
Operating expenses:
Research and development	91,461	111,209	75,595
General and administrative	28,372	23,900	20,866
Total operating expenses	119,833	135,109	96,461
Operating loss	(111,826)	(132,549)	(55,050)
Other income, net:
Interest income, net	1,531	94	95
Other income, net	83	79	59
Other income, net	1,614	173	154
Loss before income taxes	(110,212)	(132,376)	(54,896)
Income tax expense	0	0	109
Net loss	(110,212)	(132,376)	(55,005)
Other Comprehensive loss:
Unrealized loss on available for sale securities	(106)
Comprehensive loss	$ (110,318)	$ (132,376)	$ (55,005)
Loss per share allocable to common stockholders:
Basic	$ (1.93)	$ (3.32)	$ (1.67)
Diluted	$ (1.93)	$ (3.32)	$ (1.67)
Weighted average shares outstanding:
Basic	57,126	39,839	33,027
Diluted	57,126	39,839	33,027

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (110,212)	$ (132,376)	$ (55,005)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development		40,000
Depreciation and amortization	1,589	1,419	742
Stock-based compensation	10,568	9,849	6,864
Amortization of discount on investments	(51)
Loss on disposal of property and equipment		27	2
Changes in operating assets and liabilities:
Accounts receivable	239	1,813	31,592
Prepaid expenses and other current assets	(1,542)	(1,638)	(419)
Accounts payable	341	(3,647)	3,611
Accrued expenses	4,673	4,292	411
Deferred revenue	(1,900)	7,558	(23,721)
Restricted cash and other assets	106	(178)	606
Other long-term liabilities	(182)	(44)	(46)
Net cash used in operating activities	(96,371)	(72,925)	(35,363)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available for sale securities	(229,887)
Proceeds from sales/maturities of available for sale securities	65,097
Acquisition of in-process research and development		(40,000)
Purchases of property and equipment	(624)	(183)	(2,216)
Net cash used in investing activities	(165,414)	(40,183)	(2,216)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from public offering of common stock, net of commissions	130,438	130,712	101,283
Payment of common stock offering costs	(483)	(367)	(649)
Proceeds from reimbursement of common stock offering costs			269
Payment under capital lease obligation	(561)	(441)
Proceeds from stock options exercised	1,589	996	2,736
Issuance of shares under employee stock purchase plan	374	436	454
Excess tax benefit from stock option plan			17
Net cash provided by financing activities	131,357	131,336	104,110
Net (decrease) increase in cash and cash equivalents	(130,428)	18,228	66,531
Cash and cash equivalents, beginning of period	208,323	190,095	123,564
Cash and cash equivalents, end of period	$ 77,895	208,323	190,095
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Equipment acquired under capital lease		$ 1,732
Cash paid for income taxes			$ 963

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance, Value at Dec. 31, 2013	$ 104,313	$ 3		$ 160,390	$ (56,080)
Beginning Balance, Shares at Dec. 31, 2013		28,494,447
Issuance of common stock (net of commissions and offering costs), Value	100,903			100,903
Issuance of common stock (net of commissions and offering costs), Shares		3,673,901
Exercise of stock options, Value	2,736			2,736
Exercise of stock options, Shares		2,239,643
Stock-based compensation	6,864			6,864
Excess tax benefit from stock option plan	17			17
Issuance of shares under employee stock purchase plan, Value	454			454
Issuance of shares under employee stock purchase plan, Shares		18,021
Net loss	(55,005)				(55,005)
Ending Balance, Value at Dec. 31, 2014	160,282	$ 3		271,364	(111,085)
Ending Balance, Shares at Dec. 31, 2014		34,426,012
Issuance of common stock (net of commissions and offering costs), Value	130,345	$ 1		130,344
Issuance of common stock (net of commissions and offering costs), Shares		6,701,448
Exercise of stock options, Value	996			996
Exercise of stock options, Shares		634,760
Stock-based compensation	9,849			9,849
Issuance of shares under employee stock purchase plan, Value	436			436
Issuance of shares under employee stock purchase plan, Shares		23,554
Net loss	(132,376)				(132,376)
Ending Balance, Value at Dec. 31, 2015	169,532	$ 4		412,989	(243,461)
Ending 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	788,000	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 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

Consolidated Statements of Sto7

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Stockholders' Equity [Abstract]
Issuance of common stock, commissions and offering costs	$ 483	$ 367	$ 380

The Company

The Company	12 Months Ended
	Dec. 31, 2016
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 clinical stage biopharmaceutical company that discovers, develops and plans to commercialize novel epigenetic therapies for patients with cancer and other diseases. The Company’s lead product candidate, tazemetostat, is a potent and selective inhibitor of the EZH2 HMT, an enzyme that plays an important role in various cancers. The Company owns the global development and commercialization rights to tazemetostat outside of Japan. Eisai Co. Ltd (“Eisai”) holds the rights to develop and commercialize tazemetostat in Japan, and holds a limited right of first negotiation for the rest of Asia. The Company has additional programs in development, including pinometostat, a clinical program that is subject to a collaboration with Celgene Corporation and Celgene RIVOT Ltd., an affiliate of Celgene Corporation (“Celgene”) (refer to Note 9, Collaborations Collaborations Through December 31, 2016, the Company has raised an aggregate of $728.7 million to fund its operations, of which $207.8 million was non-equity funding through its collaboration agreements, $444.9 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, 2016, the Company had $242.2 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 $353.7 million through December 31, 2016 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 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 pilot-scale manufacturing to large-scale production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
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, U.S. government agency securities, and certificates of deposit. Available for sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (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). 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 following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297 The Company did not hold any available for sale securities prior to the second quarter of 2016. Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. The amortized cost of available for sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2016, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available for sale marketable securities. There was less than $0.1 million of realized losses recognized on the sale of available for sale securities during the year ended December 31, 2016. 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, 2016 was $128.8 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of December 31, 2016 was $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, 2016. The weighted average maturity of the Company’s portfolio was approximately three months at December 31, 2016. Fair Value Measurements The Financial Accounting Standards Board (“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, 2016 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. The Company’s financial instruments as of December 31, 2015 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of December 31, 2016 and December 31, 2015, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — Fair Value as of December 31, 2015 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 197,023 $ 197,023 $ — $ — Total $ 197,023 $ 197,023 $ — $ — 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 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 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. Going Concern The Company adopted the requirements of FASB Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties About an Entity’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, 2016, 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, 2016 or 2015, 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. Acquired In-Process Research and Development The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. In the first quarter of 2015, the Company accounted for the upfront payment paid to Eisai Co., Ltd. (“Eisai”) in connection with the amended and restated collaboration and license agreement, pursuant to which the Company reacquired the worldwide rights, excluding Japan, to tazemetostat, as in-process research and development expense. 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 - 20 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 2016, 2015 or 2014. Income Taxes The Company records deferred income taxes to recognize the effect of temporary differences between tax and financial statement reporting. The Company calculates the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made. The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense. Refer to Note 6, Income Taxes Revenue Recognition The Company recognizes revenue when all of the following criteria are 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 multiple-element revenue arrangements generally include the following: • Exclusive Licenses • Research and Development Services • Option Arrangements The accounting for option arrangements is dependent on the nature of the options granted to the collaboration partner. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the options to secure exclusive licenses. Factors that the Company considers in evaluating whether options are substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the options, the cost to exercise the options relative to the total upfront consideration and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. For arrangements under which the option to secure licenses is considered substantive, the Company does not consider the licenses to be deliverables at the inception of the arrangement. For arrangements under which the option to secure licenses is not considered substantive, the Company considers the license to be a deliverable at the inception of the arrangement and, upon delivery of the license, would apply 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 the Company’s collaborative arrangements have been determined to be substantive. Milestone Revenue. At the inception of each arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (i) the entity’s performance to achieve the milestone or (ii) 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; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates 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. The Company generally considers non-refundable preclinical research and development, clinical development and regulatory milestones that the Company expects to be achieved as a result of the Company’s efforts during the period of the Company’s performance obligations under the collaboration and license agreements to be substantive and recognizes them as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If not considered to be substantive, the Company initially defers milestones and recognizes them over the remaining term of the Company’s performance obligations. Milestones that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are generally achieved after the period of the Company’s performance obligations and 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. 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 drug 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 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, less estimated forfeitures, on a straight-line basis over the requisite service period of the awards. The Company estimates forfeitures at the time of grant and revises those estimates in subsequent periods if actual forfeitures differ from those estimates. Refer to Note 10, Employee Benefit Plans 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 and, prior to its automatic conversion, redeemable convertible preferred stock participate in any 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 The Company operates as one reportable business segment: the discovery and development of novel epigenetic therapies for patients with cancer and other diseases. Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue From Contracts With Customers Revenue Recognition In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”) In November 2016, the FASB issued ASU 2016-18, Restricted Cash

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Property and Equipment, net	3. Property and Equipment, net Property and equipment, net consists of the following: December 31, 2016 2015 (In thousands) Laboratory equipment $ 3,691 $ 3,468 Computer and office equipment, furniture (1) 4,495 4,848 Leasehold improvements 464 473 Property and equipment 8,650 8,789 Less: accumulated depreciation and amortization (5,526 ) (4,700 ) Property and equipment, net $ 3,124 $ 4,089 (1) In 2015, the Company acquired $1.7 million of computer hardware and equipment, pursuant to a capital lease, the term of which expires in February 2018. Accumulated depreciation related to these assets totaled $1.1 million and $0.5 million as of December 31, 2016 and 2015, respectively. Depreciation and amortization expense was $1.6 million, $1.4 million and $0.7 million for the years ended December 31, 2016, 2015 and 2014, respectively.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 (In thousands) Prepaid clinical and manufacturing costs $ 3,666 $ 2,400 Interest receivable on available for sale securities 295 — Other prepaid expenses and other receivables 2,496 2,078 Total prepaid expenses and other current assets $ 6,457 $ 4,478

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2016
Payables and Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following: December 31, 2016 2015 (In thousands) Employee compensation and benefits $ 4,100 $ 3,314 Research and development expenses 10,925 6,518 Professional services and other 982 1,503 Accrued expenses $ 16,007 $ 11,335

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company’s losses before income taxes consist solely of domestic losses. The Company did not have income tax expense for the years ended December 31, 2016 and 2015. Income tax expense for the year ended December 31, 2014 reflects adjustments identified in 2014 related to the year ended December 31, 2013 in the course of preparing the 2013 income tax returns. The Company had no deferred income tax expense for the years ended December 31, 2016, 2015 or 2014. A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: Year Ended December 31, 2016 2015 2014 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State income taxes 4.5 5.2 4.9 Research and development and other tax credits 3.3 1.8 13.6 Permanent items (1.9 ) (1.2 ) (5.2 ) Change in valuation allowance (40.1 ) (40.0 ) (44.3 ) Return-to-provision adjustments — — (3.1 ) Change in deferred taxes 0.2 0.2 — Other 0.0 — (0.1 ) Effective income tax rate 0.0 % 0.0 % (0.2 )% Deferred Tax Assets (Liabilities) The Company’s deferred tax assets (liabilities) consist of the following: December 31, 2016 2015 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 94,390 $ 57,005 Research and development and other credit carryforwards 18,055 14,112 Capitalized start-up costs 1,899 2,111 Capitalized research and development costs 154 258 Deferred revenue 11,231 8,475 Accruals and allowances 1,435 1,233 Eisai license payment 14,035 15,155 Other 5,607 4,275 Gross deferred tax assets 146,806 102,624 Deferred tax asset valuation allowance (146,548 ) (102,370 ) Total deferred tax assets 258 254 Deferred tax liabilities: Depreciation and other (258 ) (254 ) Total deferred tax liabilities (258 ) (254 ) Net deferred tax asset (liability) $ — $ — The Company evaluated the expected recoverability of its net deferred tax assets as of December 31, 2016 and 2015 and determined that 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 has provided a full valuation allowance against its net deferred tax asset balance as of December 31, 2016 and 2015. The valuation allowance increased by $44.2 million in 2016 compared to 2015. As of December 31, 2016, the Company had operating loss carryforwards of approximately $305.6 million and $302.5 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 expire commencing in 2029. The state tax operating loss carryforwards expire commencing in 2030. Additionally, as of December 31, 2016 the Company had research and development tax credit carryforwards of approximately $5.2 million and $1.9 million available to be used as a reduction of federal income taxes and state income taxes, respectively, which expire at various dates from 2024 through 2036, as well as federal orphan drug tax credit carryforwards of $11.6 million, which would expire at various dates from 2033 through 2036, and a $0.4 million federal alternative minimum tax credit carryforward, which comprises approximately $0.4 million related to excess tax benefits that have been included in the gross deferred tax asset reflected for research and development and other credit carryforwards. This amount will be recorded as an increase to additional paid-in capital on the consolidated balance sheet when the excess benefits are realized. 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 (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, 2016 2015 (In thousands) Unrecognized tax benefits - as of beginning of year $ 3,480 $ 3,062 Gross increases - tax positions of prior periods — (43 ) Gross increases - current period tax positions 726 461 Unrecognized tax benefits - as of end of year $ 4,206 $ 3,480 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, 2016 or 2015. The Company files income tax returns in the U.S. federal tax jurisdiction and Massachusetts, North Carolina and Colorado state tax jurisdictions. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Commitments The Company leases office and laboratory space at Technology Square in Cambridge, Massachusetts under an operating lease agreement with a term through May 31, 2018, with an option to extend the term of the lease for an additional five-year period at the then-current market rent, as defined in the lease. The lease extension notice date is August 31, 2017. An extension fee of $0.2 million will be due and payable on November 30, 2017 if the Company has not (i) exercised its five-year option to renew the headquarter lease or (ii) signed a lease with its existing landlord for a new location with increased square footage. 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, inclusive of 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 $2.8 million, $2.7 million and $2.5 million for the years ended December 31, 2016, 2015, and 2014, respectively. The Company’s contractual commitments under these leases, excluding common area maintenance charges and real estate taxes, as of December 31, 2016 are as follows: Total 2017 2018 2019 (In thousands) Operating leases $ 4,100 $ 2,898 $ 1,202 $ — Capital lease, including amounts representing interest 776 665 111 — Total commitments $ 4,876 $ 3,563 $ 1,313 $ — In addition to commitments under leasing arrangements, as of December 31, 2016 the Company has committed to $12.0 million of remaining development costs payable to Roche Molecular, expected to be paid through 2019 upon certain development and regulatory milestones, under an amended companion diagnostic agreement. 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. Contingencies In October 2013, the Company entered into a license agreement with a third party to obtain a non-exclusive license to a patent related to an excipient in the formulation of pinometostat. During the term of this license agreement, the Company may be required to make a €0.3 million milestone payment upon the first approval of a new drug application for pinometostat and pay royalties in the low single digits on commercial net sales of pinometostat.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Stockholders' (Deficit) Equity	8. 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 January 2016, the Company issued an additional 15,333,334 shares of common stock in connection with a public offering. The issuance of these shares contributed to a significant increase in the Company’s shares outstanding as of December 31, 2016 and in the weighted average shares outstanding for the year ended December 31, 2016 when compared to the comparable prior year periods. During the year ended December 31, 2016, the Company also sold 23,581 shares of Common Stock under an “at the market” program (“ATM Facility”) with Cowen and Company, LLC (“Cowen”) acting as sales agent during the year ended December 31, 2016. Cowen was compensated at a fixed commission rate of 3.0%. Transactions under the ATM Facility resulted in net proceeds of $0.3 million in the year ended December 31, 2016. The Company also incurred other issuance related costs of $0.1 million associated with the ATM Facility in the fourth quarter of 2016, 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, 2016, the Company sold 155,834 shares of Common Stock under the sales agreement with Cowen, resulting in net proceeds of $1.9 million related to the ATM Facility. The Company terminated the sales agreement with Cowen, effective March 10, 2017. As of December 31, 2016, a total of 6,952,359 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, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	9. 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 (“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 of global development co-funding through December 31, 2016. 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 (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 retains its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, • Celgene’s other option rights have been 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 have been 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 has been extended for each of the Option Targets and Celgene’s option is exercisable at the time of the Company’s investigational new drug application (“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 have been 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 have been 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 determined that the amended and restated agreement represented a modification of the original agreement. Accordingly, the Company determined that the remaining deferred revenue of $21.6 million attributable to the original agreement should be combined with the $10.0 million upfront payment received from Celgene upon execution of the amended and restated agreement. This combined amount, along with any development milestone-based license fees associated with those Option Targets where Celgene’s option rights are determined to be non-substantive, will be recognized on a prospective basis after (i) identifying the deliverables and units of accounting included in the amended and restated agreement, (ii) allocating the total arrangement fee among the units of accounting and (iii) determining the revenue recognition for each unit of accounting. The Company evaluated the deliverables in the amended and restated agreement and determined that the deliverables included its obligation to provide research and development services for DOT1L and the three Option Targets. In addition, the Company concluded that Celgene’s option rights are not substantive and the licenses to HMT inhibitors targeting each Option Target were therefore considered to be deliverables. As a result, the Company has determined the deliverables in the amended and restated arrangement to be: • Research and development services for DOT1L and the three Option Targets • Licenses to HMT inhibitors targeting the Option Targets The Company evaluated the nature of the deliverables to determine the units of accounting. At a program level (that is, for DOT1L and each Option Target), the Company believes there is standalone value. Therefore, the identified deliverables were evaluated on a program by program basis to determine the units of accounting. The Company determined the research services related to DOT1L have stand-alone value as the services are delivered and therefore represent a single unit of accounting. There are no other deliverables related to the DOT1L program. For two of the Option Targets, the Company concluded the licenses and related research services should be accounted for as a combined unit of accounting up until an IND filing. Prior to an IND filing, the delivered research services do not have standalone value from the remaining undelivered elements because Celgene could not resell the research services and no other vendor has the specialized skills and know-how related to HMT inhibitors necessary to provide the research services. After an IND filing, for these two Option Targets, the Company concluded the delivered pre-IND research services and licenses, once delivered, would have standalone value from the remaining research services because Celgene, or other market participants, would have the ability to execute human clinical trials on the identified compound. After an IND filing, the remaining research services represent a separate unit of accounting which has standalone value upon delivery. With respect to the third Option Target where the Company retains U.S. rights, because the license terms restrict Celgene’s access to information through the completion of a Phase 1 trial that would otherwise be necessary for Celgene, or other market participants, to execute human clinical trials on the identified compound, the Company determined the licenses did not have standalone value apart from the research services Accordingly, for the third Option Target, the licenses and research services will be accounted for as a combined unit of accounting. Because Celgene’s option rights with respect to the Option Targets are not considered substantive, any development milestone-based license fees associated with the Option Targets would be considered to be part of the total consideration for purposes of allocating the arrangement consideration. Accordingly, the Company has identified the total allocable arrangement consideration to be $106.6 million, as follows: (i) the remaining $21.6 million of deferred revenue under the original agreement, (ii) the $10.0 million upfront payment made in connection with the amended and restated agreement, and (iii) the $75.0 million of aggregate development milestone-based license fees associated with the Option Targets. The allocable arrangement consideration has been allocated to the identified deliverables using the relative selling price method. The Company estimated the selling price of the license deliverable for each Option Target using management’s best estimate of selling price after considering potential future cash flows under each license and industry benchmarks on the probability of achieving development milestones. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the selling price of the research services to be provided in connection with the DOT1L Phase 1 clinical trial and related to each Option Target using management’s best estimate of selling price based on the Company’s estimated cost of providing the services plus an applicable profit margin of 10%, which is commensurate with observable market data for similar services. Due to the stage of development of each of the option programs and considering the expected timing of delivery for those deliverables associated with each Option Target, the Company does not expect to recognize revenue related to the Option Targets in 2017. The Company expects to recognize the allocated arrangement consideration as follows, subject to the limitation described in ASC 605-25-30-5: • Research and development services for DOT1L: The arrangement consideration allocated to the DOT1L program will be recognized as the research and development services are performed. The Company allocated $2.8 million to the remaining research services under the DOT1L program, and this amount was recognized through December 31, 2016, the period over which the related research services were provided. • Two Option Targets: The arrangement consideration allocated to these Option Targets of $70.2 million in the aggregate will be deferred until an IND filing. The Company does not expect to recognize any revenue related to the Option Targets during 2017. • Third Option Target: The licenses and the research services provided through completion of a Phase 1 trial represent a single combined unit of accounting. Therefore, because a license is the last delivered item in this unit of accounting, the Company will defer all consideration allocated to this unit of accounting, $33.6 million, until the delivery of the license upon completion of a Phase 1 clinical trial. The Company does not expect to recognize any revenue related to the Option Target during 2017. • If any of the programs are terminated in accordance with the amended and restated agreement, any remaining deferred revenue will be reallocated among the undelivered items based upon their relative selling prices. Collaboration Revenue Through December 31, 2016, in addition to amounts allocated to Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock, the Company had recorded a total of $110.0 million in cash and accounts receivable under the Celgene agreement, as amended, including the $3.0 million implied premium on Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock. Through December 31, 2016, the Company has recognized $74.3 million of total collaboration revenue since the inception of the collaboration, including $1.9 million, $1.1 million and $9.6 million in the years ended December 31, 2016, 2015, and 2014, respectively. In addition, the Company has recognized $0.1 million, $1.1 million and $3.9 million of total global development co-funding as a reduction to research and development expense in the years ended December 31, 2016, 2015, and 2014, respectively. As of December 31, 2016 and December 31, 2015, the Company had deferred revenue of $28.8 million and $30.7 million, respectively, related to this agreement. 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. 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 recorded a $20.0 million upfront payment, a $3.0 million payment upon the execution of the March 2014 agreement amendment, $6.0 million of fixed research funding, $15.0 million of preclinical research and development milestone payments and $9.0 million for research and development services. In addition, in the third quarter of 2016, the Company recognized a $6.0 million clinical milestone following GSK’s initiation of patient dosing in a Phase 1 clinical trial of a PRMT5 inhibitor that was discovered by the Company and licensed to GSK. The Company is eligible to receive up to $18.0 million in additional preclinical research and development milestone payments, up to $103.0 million in clinical development milestone payments, up to $278.0 million in regulatory milestone payments and up to $218.0 million in sales-based milestone payments. In addition, GSK is required to pay the Company royalties, at percentages from the mid-single digits to the low double-digits, on a licensed product-by-licensed product basis, on worldwide net product sales, subject to reduction in specified circumstances. 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 GSK. The next potential milestone that the Company might receive under this agreement would be a $10.0 million payment for the commencement of a good laboratory practices, or GLP, toxicity study by GSK for the second licensed target under the agreement. GSK became solely responsible for development and commercialization for each licensed target in the collaboration when the research term ended on January 8, 2015. Accounting Analysis. The significant deliverables of this multiple-element revenue arrangement were determined to be exclusive licenses to three targets and corresponding research services for each target. At the inception of the arrangement, the Company concluded that the licenses could not be used for their intended purpose without the highly specialized skills and know-how relating to HMT inhibitors that is only available from the Company. The Company therefore concluded that the target licenses lacked standalone value apart from the related research services due to the limited economic benefit that GSK would derive from the licenses if it did not obtain the Company’s research services and due to the lack of transferability of the exclusive licenses. The Company therefore accounted for these deliverables, on a license-by-license basis, as a combined unit of accounting. The Company concluded that the option to secure licenses for three targets was not substantive, as the Company was not at risk with regard to GSK exercising its option due to the size of the upfront payment and the research funding commitment. Since the option was not considered substantive, the Company considered the licenses to be deliverables at the inception of the agreement. While the Company concluded that there were three units of accounting, each consisting of a license to a target and the research and development services related to that target, because the targets were at similar stages of development at the inception of the agreement, had equal probabilities of success and the research services in each unit of accounting were initially expected to be performed concurrently on a ratable basis over the research term, the Company allocated the arrangement consideration equally across the three targets. Accordingly, the $30.0 million of allocable arrangement consideration, consisting of the $20.0 million upfront payment, $4.0 million in milestone payments achieved during the selection term and the $6.0 million fixed research funding, was recognized as collaboration revenue, on a target-by-target basis, ratably from the conclusion of the selection term, in July 2012, through the end of the research term, or earlier if a target reaches development candidate selection, at which point GSK is solely responsible for development and commercialization. In December 2013, the Company and GSK agreed to the selection of a development candidate for one of the three targets under the agreement, earning the Company a $4.0 million milestone payment and reducing the period over which the Company was recognizing revenue for this target by nine months. Accordingly, the Company recognized the remaining deferred revenue related to this target during the first quarter of 2014. As to this target, GSK is solely responsible for subsequent development and commercialization. The $3.0 million upfront payment received in connection with the March 2014 amendment was allocated equally to the remaining two targets for which the Company was actively providing research and development services, as these remaining two targets were at similar stages of development, had equal probabilities of success and the remaining research services were expected to be performed concurrently on a ratable basis over the research term. The $3.0 million was recognized as collaboration revenue, on a target-by-target basis, ratably from the execution of the amendment, in March 2014, through the end of the research term. In the fourth quarter of 2014, the Company agreed to perform additional preclinical research and development studies related to the third target under the agreement, which extended the research term for this target through June 2015. Accordingly, the Company recognized the remaining deferred revenue related to this deliverable, of approximately $1.2 million as of December 31, 2014, in 2015. During the selection term, the Company received $4.0 million upon the achievement of preclinical research and development milestones which required effort in the form of research activities by the Company and was not certain to be achieved at the execution of the agreement. However, because GSK had the right to drop a target and select a replacement target at any point during the selection term, the Company, in such a case, would have been obligated to perform the validation work for a replacement target. Consequently, this $4.0 million in preclinical research and development milestones was combined with the upfront payment and fixed research funding and was recognized as collaboration revenue ratably over the research term. The Company has evaluated the remaining milestones under this agreement and determined that the milestones through development candidate selection are substantive given the significant uncertainty as to the outcome of the substantial research efforts to be performed by the Company in order to achieve the milestones and will be recognized as revenue upon achievement, assuming all other revenue recognition criteria are met. The milestones after development candidate selection are not considered substantive because the Company does not contribute effort to the achievement of such milestones, which would generally be achieved after the research term. In 2014, the Company achieved $3.0 million in preclinical research and development milestones upon the selection of lead candidates for the second and third targets under the agreement. In the fourth quarter of 2013, the Company achieved a $4.0 million preclinical research and development milestone upon the selection of a development candidate for one of the three targets under the agreement. In the third quarter of 2012, the Company achieved two additional preclinical research and development milestones and received payments totaling $4.0 million. The preclinical research and development milestones achieved in 2014, 2013 and 2012 required effort in the form of research activities by the Company and were not certain to be achieved at the execution of the agreement. Additionally, at the time of the achievement of these preclinical research and development milestones, the selection term had expired and, as such, these milestones were determined to be substantive, and the milestones were recognized as revenue upon achievement. Agreement Termination Rights. GSK has the right to terminate the agreement at any time with respect to one or more selected targets or in its entirety, upon 90 days’ prior written notice to the Company. The agreement may also be terminated with respect to one or more selected targets or in its entirety by either GSK or the Company in the event of a material breach by the other party. The agreement may be terminated with respect to selected targets by the Company in the event GSK, or an affiliate or sublicensee of GSK, participates or actively assists in a legal challenge to one of the patents exclusively licensed to GSK under the agreement with respect to the applicable target. Collaboration Revenue Through December 31, 2016, the Company has earned a total of $59.0 million under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss, including $6.0 million, $1.4 million and $25.5 million in the years ended December 31, 2016, 2015 and 2014, respectively. The Company did not have any deferred revenue related to this agreement as of December 31, 2016 or 2015 and any future revenues will relate to any milestone payments and royalties received under the agreement, if any 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. 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, up to $50.0 million in regulatory milestone payments and royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan. The Company is eligible to receive from Eisai royalties at a percentage in the mid-teens on net sales of any EZH2 product in Japan. LYSA In May 2016, the Company entered into a collaboration agreement with the Lymphoma Academic Research Organisation (“LYSARC”), for the first planned combination trial of tazemetostat. LYSARC is the operational arm of the Lymphoma Study Association (“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 front-line combination treatment for diffuse large B-cell lymphoma (DLBCL), as a front-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, Tecentriq (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. Companion Diagnostics Roche Molecular. 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.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee Benefit Plans	10. 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. Incentive and non-qualified stock options expire ten years from the date of grant. 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. Stock-Based Compensation Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2016 2015 2014 (In thousands) Research and development $ 5,352 $ 5,155 $ 3,299 General and administrative 5,216 4,694 3,565 Total $ 10,568 $ 9,849 $ 6,864 Stock Options The Company uses the Black-Scholes option-pricing model to measure the fair value of stock option awards. Key assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2016 2015 2014 Risk-free interest rate 1.2 % 1.6 % 1.6 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 78.5 % 83.6 % 92.1 % Expected dividend yield 0.0 % 0.0 % 0.0 % There were no stock option awards granted to non-employees in 2016, 2015 or 2014. 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, 2016: Number of Weighted Weighted Aggregate (In thousands) (In years) (In thousands) Outstanding at December 31, 2015 3,100 $ 15.20 Granted 2,213 9.50 Exercised (788 ) 2.02 Forfeited or expired (466 ) 18.08 Outstanding at December 31, 2016 4,059 $ 14.32 8.4 $ 8,724 Exercisable at December 31, 2016 1,162 $ 18.39 6.9 $ 3,106 Vested and expected to vest 3,695 $ 14.53 8.3 $ 8,001 During the years ended December 31, 2016, 2015 and 2014, the Company granted stock options to purchase an aggregate of 2,212,668 shares, 1,289,755 shares and 876,385 shares, respectively, at weighted average grant date fair values per option share of $6.42, $14.57 and $21.73, respectively. The total grant date fair value of options that vested during the years ended December 31, 2016, 2015 and 2014 was $12.0 million, $9.3 million and $4.9 million, respectively. The aggregate intrinsic value of stock options exercised was $7.1 million in 2016, $11.9 million in 2015 and $59.4 million in 2014. As of December 31, 2016, there was $19.2 million of unrecognized stock-based compensation related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.8 years. Restricted Stock The following is a summary of restricted stock unit activity for the year ended December 31, 2016: Number of Weighted (In thousands) Outstanding at December 31, 2015 37 $ 18.49 Granted 81 9.29 Vested (54 ) 12.20 Outstanding at December 31, 2016 64 $ 12.20 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 the Company’s chief financial officer’s employment agreement. One quarter of these restricted stock units vested on February 9, 2016 and the remaining three quarters are vesting on a straight line basis over 36 months. As of December 31, 2016 there was $0.8 million of unrecognized compensation cost related to restricted stock units that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.1 years. The intrinsic value of restricted stock that vested during the years ended December 31, 2016, 2015 and 2014 was $0.5 million, $0.0 million and $0.1 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 $0.4 million, $0.4 million and $0.2 million in the years ended December 31, 2016, 2015 and 2014, respectively.

Loss per Share

Loss per Share	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Loss per Share	11. 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, 2016 2015 2014 (In thousands except per share data) Net loss $ (110,212 ) $ (132,376 ) $ (55,005 ) Weighted average shares outstanding 57,126 39,839 33,027 Basic and diluted loss per share allocable to common stockholders $ (1.93 ) $ (3.32 ) $ (1.67 ) 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, 2016 2015 2014 (In thousands) Stock options 4,059 3,100 2,960 Unvested restricted stock 64 37 — Shares issuable under employee stock purchase plan 46 8 6 4,169 3,145 2,966

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions In connection with its entry into the collaboration agreement with Celgene, on April 2, 2012, the Company sold Celgene 9,803,922 shares of its Series C Preferred Stock. Refer to Note 9, Collaborations Under the Celgene collaboration agreement, the Company recognized $1.9 million, $1.1 million and $9.6 million of collaboration revenue in the years ended December 31, 2016, 2015 and 2014, respectively, and as of December 31, 2016 and 2015, had recorded $28.8 million and $30.7 million of deferred revenue related to the Celgene collaboration arrangement, respectively. Additionally, in the years ended December 31, 2016, 2015 and 2014, the Company recorded $0.1 million, $1.1 million and $3.9 million, respectively, in global development co-funding

Unaudited Quarterly Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Results	13. Unaudited Quarterly Results The results of operations on a quarterly basis for the years ended December 31, 2016 and 2015 are set forth below: Quarter Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Collaboration revenue $ 472 $ 473 $ 6,584 $ 478 Operating expenses: Research and development 17,740 21,450 23,888 28,383 General and administrative 5,846 7,424 7,522 7,580 Total operating expenses 23,586 28,874 31,410 35,963 Operating loss (23,114 ) (28,401 ) (24,826 ) (35,485 ) Other income, net 235 420 490 469 Net loss $ (22,879 ) $ (27,981 ) $ (24,336 ) $ (35,016 ) Loss per share allocable to common stockholders: Basic $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Diluted $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Weighted average shares outstanding: Basic 55,149 57,352 57,970 58,016 Diluted 55,149 57,352 57,970 58,016 Quarter Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Collaboration revenue $ 911 $ 736 $ 358 $ 555 Operating expenses: Research and development 57,051 20,551 16,788 16,819 General and administrative 5,237 5,970 6,676 6,017 Total operating expenses 62,288 26,521 23,464 22,836 Operating loss (61,377 ) (25,785 ) (23,106 ) (22,281 ) Other income, net 51 26 41 55 Net loss $ (61,326 ) $ (25,759 ) $ (23,065 ) $ (22,226 ) Loss per share allocable to common stockholders: Basic $ (1.75 ) $ (0.63 ) $ (0.56 ) $ (0.53 ) Diluted $ (1.75 ) $ (0.63 ) $ (0.56 ) $ (0.53 ) Weighted average shares outstanding: Basic 34,992 41,087 41,461 41,725 Diluted 34,992 41,087 41,461 41,725

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
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, U.S. government agency securities, and certificates of deposit. Available for sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (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). 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 following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297 The Company did not hold any available for sale securities prior to the second quarter of 2016. Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents within the consolidated balance sheets and are not included in the tables above. The amortized cost of available for sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2016, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available for sale marketable securities. There was less than $0.1 million of realized losses recognized on the sale of available for sale securities during the year ended December 31, 2016. 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, 2016 was $128.8 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of December 31, 2016 was $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, 2016. The weighted average maturity of the Company’s portfolio was approximately three months at December 31, 2016.
Fair Value Measurements	Fair Value Measurements The Financial Accounting Standards Board (“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, 2016 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. The Company’s financial instruments as of December 31, 2015 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of December 31, 2016 and December 31, 2015, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — Fair Value as of December 31, 2015 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 197,023 $ 197,023 $ — $ — Total $ 197,023 $ 197,023 $ — $ — 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 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 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.
Going Concern	Going Concern The Company adopted the requirements of FASB Accounting Standards Update (“ASU”) No. 2014-15, Disclosure of Uncertainties About an Entity’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, 2016, 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, 2016 or 2015, 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.
Acquired In-Process Research and Development	Acquired In-Process Research and Development The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use. In the first quarter of 2015, the Company accounted for the upfront payment paid to Eisai Co., Ltd. (“Eisai”) in connection with the amended and restated collaboration and license agreement, pursuant to which the Company reacquired the worldwide rights, excluding Japan, to tazemetostat, as in-process research and development expense.
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 - 20 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 2016, 2015 or 2014.
Income Taxes	Income Taxes The Company records deferred income taxes to recognize the effect of temporary differences between tax and financial statement reporting. The Company calculates the deferred taxes using enacted tax rates expected to be in place when the temporary differences are realized and records a valuation allowance to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made. The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense. Refer to Note 6, Income Taxes
Revenue Recognition	Revenue Recognition The Company recognizes revenue when all of the following criteria are 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 multiple-element revenue arrangements generally include the following: • Exclusive Licenses • Research and Development Services • Option Arrangements The accounting for option arrangements is dependent on the nature of the options granted to the collaboration partner. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the options to secure exclusive licenses. Factors that the Company considers in evaluating whether options are substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the options, the cost to exercise the options relative to the total upfront consideration and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. For arrangements under which the option to secure licenses is considered substantive, the Company does not consider the licenses to be deliverables at the inception of the arrangement. For arrangements under which the option to secure licenses is not considered substantive, the Company considers the license to be a deliverable at the inception of the arrangement and, upon delivery of the license, would apply 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 the Company’s collaborative arrangements have been determined to be substantive. Milestone Revenue. At the inception of each arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (i) the entity’s performance to achieve the milestone or (ii) 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; (b) the consideration relates solely to past performance; and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates 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. The Company generally considers non-refundable preclinical research and development, clinical development and regulatory milestones that the Company expects to be achieved as a result of the Company’s efforts during the period of the Company’s performance obligations under the collaboration and license agreements to be substantive and recognizes them as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If not considered to be substantive, the Company initially defers milestones and recognizes them over the remaining term of the Company’s performance obligations. Milestones that are not considered substantive because the Company does not contribute effort to the achievement of such milestones are generally achieved after the period of the Company’s performance obligations and 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.
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 drug 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, less estimated forfeitures, on a straight-line basis over the requisite service period of the awards. The Company estimates forfeitures at the time of grant and revises those estimates in subsequent periods if actual forfeitures differ from those estimates. 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 and, prior to its automatic conversion, redeemable convertible preferred stock participate in any 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.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue From Contracts With Customers Revenue Recognition In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“Topic 230”) In November 2016, the FASB issued ASU 2016-18, Restricted Cash

Summary of Significant Accoun22

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available for sale securities held at December 31, 2016 (in thousands): Description Amortized Cost Unrealized Unrealized Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297
Summary of Company's Financial Assets Recognized at Fair Value	As of December 31, 2016 and December 31, 2015, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 62,854 $ 59,862 $ 2,992 $ — Marketable securities: Commercial paper 68,375 — 68,375 — Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — Fair Value as of December 31, 2015 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 197,023 $ 197,023 $ — $ — Total $ 197,023 $ 197,023 $ — $ —
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 - 20 years Office furniture and equipment 3 - 10 years Leasehold improvements 3 - 10 years or term of respective lease, if shorter

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net consists of the following: December 31, 2016 2015 (In thousands) Laboratory equipment $ 3,691 $ 3,468 Computer and office equipment, furniture (1) 4,495 4,848 Leasehold improvements 464 473 Property and equipment 8,650 8,789 Less: accumulated depreciation and amortization (5,526 ) (4,700 ) Property and equipment, net $ 3,124 $ 4,089 (1) In 2015, the Company acquired $1.7 million of computer hardware and equipment, pursuant to a capital lease, the term of which expires in February 2018. Accumulated depreciation related to these assets totaled $1.1 million and $0.5 million as of December 31, 2016 and 2015, respectively.

Prepaid Expenses and Other Cu24

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: December 31, 2016 2015 (In thousands) Prepaid clinical and manufacturing costs $ 3,666 $ 2,400 Interest receivable on available for sale securities 295 — Other prepaid expenses and other receivables 2,496 2,078 Total prepaid expenses and other current assets $ 6,457 $ 4,478

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2016
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: December 31, 2016 2015 (In thousands) Employee compensation and benefits $ 4,100 $ 3,314 Research and development expenses 10,925 6,518 Professional services and other 982 1,503 Accrued expenses $ 16,007 $ 11,335

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
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, 2016 2015 2014 Federal statutory income tax rate 34.0 % 34.0 % 34.0 % State income taxes 4.5 5.2 4.9 Research and development and other tax credits 3.3 1.8 13.6 Permanent items (1.9 ) (1.2 ) (5.2 ) Change in valuation allowance (40.1 ) (40.0 ) (44.3 ) Return-to-provision adjustments — — (3.1 ) Change in deferred taxes 0.2 0.2 — Other 0.0 — (0.1 ) Effective income tax rate 0.0 % 0.0 % (0.2 )%
Deferred Tax Assets (Liabilities)	The Company’s deferred tax assets (liabilities) consist of the following: December 31, 2016 2015 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 94,390 $ 57,005 Research and development and other credit carryforwards 18,055 14,112 Capitalized start-up costs 1,899 2,111 Capitalized research and development costs 154 258 Deferred revenue 11,231 8,475 Accruals and allowances 1,435 1,233 Eisai license payment 14,035 15,155 Other 5,607 4,275 Gross deferred tax assets 146,806 102,624 Deferred tax asset valuation allowance (146,548 ) (102,370 ) Total deferred tax assets 258 254 Deferred tax liabilities: Depreciation and other (258 ) (254 ) Total deferred tax liabilities (258 ) (254 ) Net deferred tax asset (liability) $ — $ —
Summary of Unrecognized Tax Benefits	The following is a rollforward of the Company’s unrecognized tax benefits: December 31, 2016 2015 (In thousands) Unrecognized tax benefits - as of beginning of year $ 3,480 $ 3,062 Gross increases - tax positions of prior periods — (43 ) Gross increases - current period tax positions 726 461 Unrecognized tax benefits - as of end of year $ 4,206 $ 3,480

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 are as follows: Total 2017 2018 2019 (In thousands) Operating leases $ 4,100 $ 2,898 $ 1,202 $ — Capital lease, including amounts representing interest 776 665 111 — Total commitments $ 4,876 $ 3,563 $ 1,313 $ —

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based 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, 2016 2015 2014 (In thousands) Research and development $ 5,352 $ 5,155 $ 3,299 General and administrative 5,216 4,694 3,565 Total $ 10,568 $ 9,849 $ 6,864
Assumptions Used in Applying Pricing Model	Key assumptions used in this pricing model on the date of grant for options granted to employees are as follows: Year Ended December 31, 2016 2015 2014 Risk-free interest rate 1.2 % 1.6 % 1.6 % Expected life of options 6.0 years 6.0 years 6.0 years Expected volatility of underlying stock 78.5 % 83.6 % 92.1 % 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, 2016: Number of Weighted Weighted Aggregate (In thousands) (In years) (In thousands) Outstanding at December 31, 2015 3,100 $ 15.20 Granted 2,213 9.50 Exercised (788 ) 2.02 Forfeited or expired (466 ) 18.08 Outstanding at December 31, 2016 4,059 $ 14.32 8.4 $ 8,724 Exercisable at December 31, 2016 1,162 $ 18.39 6.9 $ 3,106 Vested and expected to vest 3,695 $ 14.53 8.3 $ 8,001
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the year ended December 31, 2016: Number of Weighted (In thousands) Outstanding at December 31, 2015 37 $ 18.49 Granted 81 9.29 Vested (54 ) 12.20 Outstanding at December 31, 2016 64 $ 12.20

Loss per Share (Tables)

Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 2014 (In thousands except per share data) Net loss $ (110,212 ) $ (132,376 ) $ (55,005 ) Weighted average shares outstanding 57,126 39,839 33,027 Basic and diluted loss per share allocable to common stockholders $ (1.93 ) $ (3.32 ) $ (1.67 )
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, 2016 2015 2014 (In thousands) Stock options 4,059 3,100 2,960 Unvested restricted stock 64 37 — Shares issuable under employee stock purchase plan 46 8 6 4,169 3,145 2,966

Unaudited Quarterly Results (Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 are set forth below: Quarter Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Collaboration revenue $ 472 $ 473 $ 6,584 $ 478 Operating expenses: Research and development 17,740 21,450 23,888 28,383 General and administrative 5,846 7,424 7,522 7,580 Total operating expenses 23,586 28,874 31,410 35,963 Operating loss (23,114 ) (28,401 ) (24,826 ) (35,485 ) Other income, net 235 420 490 469 Net loss $ (22,879 ) $ (27,981 ) $ (24,336 ) $ (35,016 ) Loss per share allocable to common stockholders: Basic $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Diluted $ (0.41 ) $ (0.49 ) $ (0.42 ) $ (0.60 ) Weighted average shares outstanding: Basic 55,149 57,352 57,970 58,016 Diluted 55,149 57,352 57,970 58,016 Quarter Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Collaboration revenue $ 911 $ 736 $ 358 $ 555 Operating expenses: Research and development 57,051 20,551 16,788 16,819 General and administrative 5,237 5,970 6,676 6,017 Total operating expenses 62,288 26,521 23,464 22,836 Operating loss (61,377 ) (25,785 ) (23,106 ) (22,281 ) Other income, net 51 26 41 55 Net loss $ (61,326 ) $ (25,759 ) $ (23,065 ) $ (22,226 ) Loss per share allocable to common stockholders: Basic $ (1.75 ) $ (0.63 ) $ (0.56 ) $ (0.53 ) Diluted $ (1.75 ) $ (0.63 ) $ (0.56 ) $ (0.53 ) Weighted average shares outstanding: Basic 34,992 41,087 41,461 41,725 Diluted 34,992 41,087 41,461 41,725

The Company - Additional Inform

The Company - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 129,955	$ 130,345	$ 100,903
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 242,200
Accumulated deficit	(353,673)	$ (243,461)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	728,700
Non-equity funding through collaboration agreement	207,800
IPO [Member]
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 444,900

Summary of Significant Accoun32

Summary of Significant Accounting Policies - Summary of Available-for-Sale Securities Held (Detail) - USD ($)	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 164,403,000
Available-For-Sale Securities Held, Unrealized Gains	7,000
Available-For-Sale Securities Held, Unrealized Losses	(113,000)
Available-For-Sale Securities Held, Fair Value	164,297,000	$ 0
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	68,407,000
Available-For-Sale Securities Held, Unrealized Losses	(32,000)
Available-For-Sale Securities Held, Fair Value	68,375,000
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	70,489,000
Available-For-Sale Securities Held, Unrealized Losses	(81,000)
Available-For-Sale Securities Held, Fair Value	70,408,000
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	25,507,000
Available-For-Sale Securities Held, Unrealized Gains	7,000
Available-For-Sale Securities Held, Fair Value	$ 25,514,000

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016USD ($)Segment	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Mar. 31, 2016USD ($)
Accounting Policies [Line Items]
Available-For-Sale Securities Held, Fair Value	$ 164,297,000			$ 0
Realized gains (losses) recognized on sale of marketable equity securities	100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	128,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 100,000
Weighted average maturity period	3 months
Allowance for doubtful accounts	$ 0	$ 0
Impairment of long-lived assets	$ 0	$ 0	$ 0
Largest amount of tax benefit	50.00%
Reportable business segment | Segment	1
Change in amount of valuation allowance and deferred tax asset	$ 44,200,000
Minimum [Member]
Accounting Policies [Line Items]
Change in amount of valuation allowance and deferred tax asset	23,000,000
Maximum [Member]
Accounting Policies [Line Items]
Change in amount of valuation allowance and deferred tax asset	$ 27,000,000

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 62,854	$ 197,023
Total	227,151	197,023
Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	68,375
Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	70,408
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	25,514
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	59,862	197,023
Total	59,862	$ 197,023
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	2,992
Total	167,289
Level 2 [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	68,375
Level 2 [Member] | Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	70,408
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	$ 25,514

Summary of Significant Accoun35

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	12 Months Ended
	Dec. 31, 2016
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	20 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

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment	$ 8,650	$ 8,789
Less: accumulated depreciation and amortization	(5,526)	(4,700)
Property and equipment, net	3,124	4,089
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	3,691	3,468
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	4,495	4,848
Less: accumulated depreciation and amortization	(1,100)	(500)
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 464	$ 473

Property and Equipment, Net -37

Property and Equipment, Net - Schedule of Property and Equipment, Net (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Expiration of capital lease term	May 31, 2018
Accumulated depreciation	$ 5,526	$ 4,700
Computer Office Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Computer hardware and equipment acquired		1,700
Expiration of capital lease term	Feb. 28, 2018
Accumulated depreciation	$ 1,100	$ 500

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 1,589	$ 1,419	$ 742

Prepaid Expenses and Other Cu39

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid clinical and manufacturing costs	$ 3,666	$ 2,400
Interest receivable on available for sale securities	295
Other prepaid expenses and other receivables	2,496	2,078
Total prepaid expenses and other current assets	$ 6,457	$ 4,478

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$ 4,100	$ 3,314
Research and development expenses	10,925	6,518
Professional services and other	982	1,503
Accrued expenses	$ 16,007	$ 11,335

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income tax expense	$ 0	$ 0	$ 109,000
Deferred income tax expense	$ 0	$ 0	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State income taxes	4.50%	5.20%	4.90%
Research and development and other tax credits	3.30%	1.80%	13.60%
Permanent items	(1.90%)	(1.20%)	(5.20%)
Change in valuation allowance	(40.10%)	(40.00%)	(44.30%)
Return-to-provision adjustments			(3.10%)
Change in deferred taxes	0.20%	0.20%
Other	0.00%		(0.10%)
Effective income tax rate	0.00%	0.00%	(0.20%)

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 94,390	$ 57,005
Research and development and other credit carryforwards	18,055	14,112
Capitalized start-up costs	1,899	2,111
Capitalized research and development costs	154	258
Deferred revenue	11,231	8,475
Accruals and allowances	1,435	1,233
Eisai license payment	14,035	15,155
Other	5,607	4,275
Gross deferred tax assets	146,806	102,624
Deferred tax asset valuation allowance	(146,548)	(102,370)
Total deferred tax assets	258	254
Deferred tax liabilities:
Depreciation and other	(258)	(254)
Total deferred tax liabilities	(258)	(254)
Net deferred tax asset (liability)

Income Taxes (Valuation Allowan

Income Taxes (Valuation Allowance) - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)
Income Tax Disclosure [Abstract]
Valuation allowance increase	$ 44.2

Income Taxes (Operating Loss Ca

Income Taxes (Operating Loss Carryforwards) - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)
United States, Federal [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 305.6
Operating loss carryforwards, expiration beginning year	2,029
United States, State [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 302.5
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, 2016	Dec. 31, 2015
Tax Credit Carryforward [Line Items]
Research and development tax credit carryforward expiration beginning year	2,024
Research and development tax credit carryforward expiration ending year	2,036
Federal orphan drug tax credit carryforwards expiration beginning year	2,033
Expiry period of federal orphan drug tax credit carryforwards	2,036
Federal orphan drug tax credit carryforwards	$ 11,600,000
Federal alternative minimum tax credit	400,000
Excess tax benefits	400,000
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	5,200,000
Research Tax Credit Carryforward [Member] | United States, State [Member]
Tax Credit Carryforward [Line Items]
Research and development credit carryforwards	$ 1,900,000

Income Taxes - Summary of Unrec

Income Taxes - Summary of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits - as of beginning of year	$ 3,480	$ 3,062
Gross increases - tax positions of prior periods		(43)
Gross increases - current period tax positions	726	461
Unrecognized tax benefits - as of end of year	$ 4,206	$ 3,480

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Oct. 31, 2013EUR (€)
Other Commitments [Line Items]
Letter of credit as a security deposit	$ 0.5
Lease expiration date	May 31, 2018
Lease term extension	5 years
Extension fee payable if lease not renewed	$ 0.2
Rental expense	2.8	$ 2.7	$ 2.5
Formulation Patent License Agreement [Member]
Other Commitments [Line Items]
Milestone payment payable | €				€ 0.3
Collaborative Arrangement [Member] | Roche [Member]
Other Commitments [Line Items]
Remaining development costs	$ 12

Commitments and Contingencies49

Commitments and Contingencies - Contractual Commitments (Detail) $ in Thousands	Dec. 31, 2016USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating lease, Total	$ 4,100
Operating lease, 2017	2,898
Operating lease, 2018	1,202
Operating lease, 2019
Capital lease, including amounts representing interest, Total	776
Capital lease, including amounts representing interest, 2017	665
Capital lease, including amounts representing interest, 2018	111
Capital lease, including amounts representing interest, 2019
Commitments, Total	4,876
Commitments, 2017	3,563
Commitments, 2018	1,313
Commitments, 2019

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity - Additional Information (Detail) $ in Thousands	Mar. 10, 2017USD ($)shares	Jan. 31, 2016shares	Dec. 31, 2016USD ($)Voteshares	Dec. 31, 2016USD ($)Voteshares	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Equity [Line Items]
Number of votes for each share of common stock | Vote			1	1
Common Stock Sold through ATM Facility | shares		15,333,334
Proceeds from Issuance of Common Stock				$ 130,438	$ 130,712	$ 101,283
Common Stock Issuance Cost Incurred				$ 483	$ 367	$ 649
Common stock reserved for future issuance | shares			6,952,359	6,952,359
At The Market Facility [Member]
Equity [Line Items]
Common Stock Sold through ATM Facility | shares				23,581
Commission rate for sales agent				3.00%
Proceeds from Issuance of Common Stock				$ 300
Common Stock Issuance Cost Incurred			$ 100
Equity offering sales agreement termination date				Mar. 10, 2017
At The Market Facility [Member] | Subsequent Event [Member]
Equity [Line Items]
Common Stock Sold through ATM Facility | shares	155,834
Proceeds from Issuance of Common Stock	$ 1,900

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015USD ($)OptionTargets	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Jun. 30, 2012USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2016USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue					$ 478,000	$ 6,584,000	$ 473,000	$ 472,000	$ 555,000	$ 358,000	$ 736,000	$ 911,000			$ 8,007,000	$ 2,560,000	$ 41,411,000
Upfront payment made																40,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days															90 days
DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payment					35,000,000										$ 35,000,000					$ 35,000,000	$ 35,000,000
DOT1L [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments					100,000,000										100,000,000					100,000,000	100,000,000
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [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
Celgene [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment																				65,000,000
Proceeds from redeemable convertible preferred stock																				25,000,000
Upfront payment recorded																				68,000,000
Clinical development milestone achieved																				25,000,000
Global development co-funding															100,000	1,100,000	3,900,000			7,000,000
Number of option targets | OptionTargets	3
Deferred revenue recognized	$ 21,600,000
Upfront payment made as part of the amendment	10,000,000
License and research services related cost	106,600,000
Cash and accounts receivable																				110,000,000
Convertible preferred stock issued, premium value																				3,000,000
Collaboration revenue															1,900,000	1,100,000	9,600,000			74,300,000
Deferred revenue					28,800,000				30,700,000						28,800,000	30,700,000				28,800,000	28,800,000
Celgene [Member] | Third Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost	33,600,000
Celgene [Member] | License and Pre-IND Services [Member] | Two Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost	$ 70,200,000
Number of option targets with no standalone value | OptionTargets	2
Celgene [Member] | Minimum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] | United States [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTargets	2
Celgene [Member] | Non-US [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets | OptionTargets	1
Celgene [Member] | DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
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
License and research services related cost	$ 2,800,000
Applicable profit margin	10.00%
Celgene [Member] | DOT1L [Member] | License and Pre-IND Services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets with standalone value | OptionTargets	2
Celgene [Member] | Available Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [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]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment															10,000,000
Development milestone and license payments associated with the Option Targets	$ 75,000,000
Celgene [Member] | Option Targets [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments					365,000,000										365,000,000					365,000,000	365,000,000
Development milestone and license payments associated with the Option Targets															75,000,000
Sales-based milestone payments					170,000,000										170,000,000					170,000,000	170,000,000
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment recorded			$ 3,000,000																$ 20,000,000
Clinical development milestone achieved						$ 6,000,000
Clinical development milestone payment					103,000,000										103,000,000					103,000,000	103,000,000
Additional milestone payments					278,000,000										278,000,000					278,000,000	278,000,000
Sales-based milestone payments					218,000,000										218,000,000					218,000,000	218,000,000
License and research services related cost																					30,000,000
Cash and accounts receivable																					59,000,000
Collaboration revenue													$ 3,000,000		6,000,000	1,400,000	25,500,000
Deferred revenue					0				0						0	0				0	0
Fixed research funding received																					6,000,000
Milestone payments received				$ 4,000,000										$ 4,000,000			3,000,000	$ 4,000,000			15,000,000
Research and development services																					9,000,000
Additional substantive preclinical research and development milestone payments					18,000,000										18,000,000					18,000,000	18,000,000
GLP toxicity study commencement milestone payments					10,000,000										$ 10,000,000					10,000,000	10,000,000
Percentage rate of research and development costs															100.00%
Upfront payment recorded									$ 1,200,000							$ 1,200,000	$ 1,200,000
Notice period in days															90 days
Collaborative Arrangement [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments					$ 12,000,000										$ 12,000,000					$ 12,000,000	$ 12,000,000

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Feb. 29, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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 and restricted stock, vesting period		4 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		2,212,668	1,289,755	876,385
Weighted-average fair value of options granted		$ 6.42	$ 14.57	$ 21.73
Grant date fair value of options vested		$ 12	$ 9.3	$ 4.9
Aggregate intrinsic value of stock option exercised		7.1	11.9	$ 59.4
Unrecognized compensation cost related to stock options		$ 19.2
Expected weighted average period for recognition of compensation cost		2 years 9 months 18 days
Percentage of employer's matching contribution				50.00%
Maximum percentage of employee contribution				3.00%
Contribution to 401 (k) plan		$ 0.4	0.4	$ 0.2
Unvested Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting terms		One quarter of these restricted stock units vested on February 9, 2016 and the remaining three quarters are vesting on a straight line basis over 36 months.
Expected weighted average period for recognition of compensation cost		2 years 1 month 6 days
Number of Shares, Granted		81,000
Weighted Average Grant Date Fair Value per Share, Granted		$ 9.29
Unrecognized compensation cost related to restricted stock units		$ 0.8
Unvested Restricted Stock [Member] | February 9, 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive and non-qualified stock options and restricted stock, vesting period		36 months
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]
Intrinsic value of restricted stock vested during period		$ 0.5	$ 0	$ 0.1
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 10,568	$ 9,849	$ 6,864
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	5,352	5,155	3,299
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 5,216	$ 4,694	$ 3,565

Employee Benefit Plans - Assump

Employee Benefit Plans - Assumptions Used in Applying Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	1.20%	1.60%	1.60%
Expected life of options	6 years	6 years	6 years
Expected volatility of underlying stock	78.50%	83.60%	92.10%
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance	3,100,000
Number of Options, Granted	2,212,668	1,289,755	876,385
Number of Options, Exercised	(788,000)
Number of Options, Forfeited or expired	(466,000)
Number of Options, Outstanding, Ending balance	4,059,000	3,100,000
Number of Options, Exercisable	1,162,000
Number of Options, Vested and expected to vest	3,695,000
Weighted Average Exercise Price per Share, Outstanding, Beginning balance	$ 15.20
Weighted Average Exercise Price per Share, Granted	9.50
Weighted Average Exercise Price per Share, Exercised	2.02
Weighted Average Exercise Price per Share, Forfeited or expired	18.08
Weighted Average Exercise Price per Share, Outstanding, Ending balance	14.32	$ 15.20
Weighted Average Exercise Price per Share, Exercisable	18.39
Weighted Average Exercise Price per Share, Vested and expected to vest	$ 14.53
Weighted Average Remaining Contractual Term (In Years), Outstanding	8 years 4 months 24 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 10 months 24 days
Weighted Average Remaining Contractual Term (In Years), Vested and expected to vest	8 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding	$ 8,724
Aggregate Intrinsic Value, Exercisable	3,106
Aggregate Intrinsic Value, Vested and expected to vest	$ 8,001

Employee Benefit Plans - Summ56

Employee Benefit Plans - Summary of Restricted Stock Activity (Detail) - Unvested Restricted Stock [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2016$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Outstanding, Beginning balance | shares	37
Number of Units, Granted | shares	81
Number of Units, Vested | shares	(54)
Number of Units, Outstanding, Ending balance | shares	64
Weighted Average Grant Date Fair Value per Unit, Beginning balance | $ / shares	$ 18.49
Weighted Average Grant Date Fair Value per Unit, Granted | $ / shares	9.29
Weighted Average Grant Date Fair Value per Unit, Vested | $ / shares	12.20
Weighted Average Grant Date Fair Value per Unit, Ending balance | $ / shares	$ 12.20

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net loss	$ (35,016)	$ (24,336)	$ (27,981)	$ (22,879)	$ (22,226)	$ (23,065)	$ (25,759)	$ (61,326)	$ (110,212)	$ (132,376)	$ (55,005)
Weighted average shares outstanding									57,126	39,839	33,027
Basic and diluted loss per share allocable to common stockholders									$ (1.93)	$ (3.32)	$ (1.67)

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,169	3,145	2,966
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,059	3,100	2,960
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	37
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	46	8	6

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended										12 Months Ended
	Jan. 31, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Mar. 31, 2014	Jun. 30, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 02, 2012
Related Party Transaction [Line Items]
Issuance of common stock	15,333,334
Equity investments in common stock		58,050,000				41,786,000						58,050,000	41,786,000
Collaboration revenue related to agreement		$ 478	$ 6,584	$ 473	$ 472	$ 555	$ 358	$ 736	$ 911			$ 8,007	$ 2,560	$ 41,411
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Issuance of common stock										340,000	66,666
Redeemable convertible preferred stock conversion ratio												One-for-three
Equity investments in common stock															3,334,640
Diluted equity		6.30%										6.30%
Collaboration revenue related to agreement												$ 1,900	1,100	9,600
Deferred revenue		$ 28,800				30,700						28,800	30,700
Global development co-funding from Celgene												100	1,100	$ 3,900
Accounts receivable related to collaboration arrangement		$ 100				$ 100						$ 100	$ 100
Beneficial Owner [Member] | Series C [Member]
Related Party Transaction [Line Items]
Redeemable convertible preferred stock, issued																9,803,922

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 478	$ 6,584	$ 473	$ 472	$ 555	$ 358	$ 736	$ 911	$ 8,007	$ 2,560	$ 41,411
Operating expenses:
Research and development	28,383	23,888	21,450	17,740	16,819	16,788	20,551	57,051	91,461	111,209	75,595
General and administrative	7,580	7,522	7,424	5,846	6,017	6,676	5,970	5,237	28,372	23,900	20,866
Total operating expenses	35,963	31,410	28,874	23,586	22,836	23,464	26,521	62,288	119,833	135,109	96,461
Operating loss	(35,485)	(24,826)	(28,401)	(23,114)	(22,281)	(23,106)	(25,785)	(61,377)	(111,826)	(132,549)	(55,050)
Other income, net	469	490	420	235	55	41	26	51	1,614	173	154
Net loss	$ (35,016)	$ (24,336)	$ (27,981)	$ (22,879)	$ (22,226)	$ (23,065)	$ (25,759)	$ (61,326)	$ (110,212)	$ (132,376)	$ (55,005)
Loss per share allocable to common stockholders:
Basic	$ (0.60)	$ (0.42)	$ (0.49)	$ (0.41)	$ (0.53)	$ (0.56)	$ (0.63)	$ (1.75)	$ (1.93)	$ (3.32)	$ (1.67)
Diluted	$ (0.60)	$ (0.42)	$ (0.49)	$ (0.41)	$ (0.53)	$ (0.56)	$ (0.63)	$ (1.75)	$ (1.93)	$ (3.32)	$ (1.67)
Weighted average shares outstanding:
Basic	58,016	57,970	57,352	55,149	41,725	41,461	41,087	34,992	57,126	39,839	33,027
Diluted	58,016	57,970	57,352	55,149	41,725	41,461	41,087	34,992	57,126	39,839	33,027