Epizyme (EPZM) 10-Q 1 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 25, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	1,571,498
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		69,282,628

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 219,794	$ 77,895
Marketable securities	87,434	164,297
Accounts receivable	25	23
Prepaid expenses and other current assets	10,000	6,457
Total current assets	317,253	248,672
Property and equipment, net	2,631	3,124
Restricted cash and other assets	662	645
Total assets	320,546	252,441
Current liabilities:
Accounts payable	5,360	4,994
Accrued expenses	17,547	16,007
Current portion of capital lease obligation	270	620
Other current liabilities	3
Total current liabilities	23,180	21,621
Capital lease obligation, net of current portion		110
Deferred revenue, net of current portion	28,809	28,809
Other long-term liabilities	368	201
Commitments and contingencies
Stockholders' equity:
Common stock $0.0001 par value; 125,000 shares authorized; 69,237 shares and 58,050 shares issued and outstanding, respectively	7	6
Additional paid-in capital	720,144	555,473
Accumulated other comprehensive loss	(31)	(106)
Accumulated deficit	(451,931)	(353,673)
Total stockholders' equity	268,189	201,700
Total liabilities and stockholders' equity	$ 320,546	$ 252,441

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
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	69,237,000	58,050,000
Common stock, shares outstanding	69,237,000	58,050,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Collaboration revenue		$ 6,584	$ 10,000	$ 7,529
Operating expenses:
Research and development	$ 28,741	23,888	80,728	63,078
General and administrative	9,311	7,522	28,750	20,792
Total operating expenses	38,052	31,410	109,478	83,870
Loss from operations	(38,052)	(24,826)	(99,478)	(76,341)
Other income, net:
Interest income, net	487	469	1,353	1,093
Other (expense) income	(32)	21	(18)	52
Other income, net	455	490	1,335	1,145
Net loss	(37,597)	(24,336)	(98,143)	(75,196)
Other comprehensive income (loss):
Unrealized gain on available-for-sale securities	29	(120)	75	(95)
Comprehensive loss	$ (37,568)	$ (24,456)	$ (98,068)	$ (75,291)
Loss per share allocable to common stockholders:
Basic	$ (0.63)	$ (0.42)	$ (1.67)	$ (1.32)
Diluted	$ (0.63)	$ (0.42)	$ (1.67)	$ (1.32)
Weighted average shares outstanding:
Basic	59,899	57,970	58,837	56,828
Diluted	59,899	57,970	58,837	56,828

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (98,143)	$ (75,196)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,230	1,189
Stock-based compensation	8,673	7,843
Amortization of discount on investments	(155)	(52)
Changes in operating assets and liabilities:
Accounts receivable	(2)	(5,938)
Prepaid expenses and other current assets	(3,350)	(564)
Accounts payable	366	(1,266)
Accrued expenses	1,187	50
Deferred revenue		(1,422)
Restricted cash and other assets	(17)	88
Other liabilities	170	(127)
Net cash used in operating activities	(90,041)	(75,395)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available-for-sale securities	(115,569)	(212,126)
Maturities of available-for-sale securities	192,469	14,350
Purchases of property and equipment	(737)	(420)
Net cash provided by (used in) investing activities	76,163	(198,196)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payments under capital lease obligation	(460)	(416)
Proceeds from public offering, net of commissions	152,920	130,067
Proceeds from stock options exercised	2,638	1,645
Issuance of shares under employee stock purchase plan	679	374
Payment of public offering costs		(374)
Net cash provided by financing activities	155,777	131,296
Net increase (decrease) in cash and cash equivalents	141,899	(142,295)
Cash and cash equivalents, beginning of period	77,895	208,323
Cash and cash equivalents, end of period	219,794	$ 66,028
SUPPLEMENTAL CASH FLOW INFORMATION:
Unpaid offering costs	353
Cumulative catch up related to the adoption of ASU 2016-09 (Note 2)	$ 115

Overview

Overview	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview	1. Overview 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 EZH2, 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 (i) 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 8, Collaborations Collaborations In September 2017, the Company raised $151.3 million in net proceeds, after underwriting discounts and commissions and before direct and incremental costs of the offering, from the sale of 10,557,000 shares of its common stock in a public offering at a price of $15.25 per share. Through September 30, 2017, the Company has raised, including amounts received under collaboration agreements, an aggregate of $891.6 million to fund its operations, of which $217.8 million was non-equity The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $451.9 million through September 30, 2017, 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 clinical-stage manufacturing to commercial-stage production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K The unaudited condensed consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. 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 three months ended September 30, 2017 and 2016 are referred to as the third quarter of 2017 and 2016, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period. Significant Accounting Policies There have been no material changes to the Company’s significant accounting policies during the nine months ended September 30, 2017, as compared to the significant accounting policies disclosed in Note 2, Summary of Significant Accounting Policies Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue From Contracts With Customers 2014-09 Revenue Recognition 2016-10 2016-12, ASU 2014-09. 2014-09 2014-09, Revenue From Contracts with Customers The Company has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements, but expects that the adoption will at least impact the amount and timing of revenues recognized related to its collaboration agreement with Celgene and the impact may be material. The Company is still assessing its collaboration agreement with Celgene; however, it currently expects that (a) the number of performance obligations identified under ASC 606 will be different from the number of units of accounting identified under ASC 605 and (b) the transaction price will be different from the transaction price determined under ASC 605. In particular, the Company currently expects that the accounting treatment for Celgene’s option rights that were determined to be non-substantive The Company also expects the accounting for contingent milestone payments under its collaboration agreements to change under ASC 606. ASC 606 does not contain guidance specific to milestone payments, thereby requiring contingent milestone payments to be considered in accordance with the overall model of ASC 606 as variable consideration. Revenue from contingent milestone payments may be recognized earlier under ASC 606 than under ASC 605, based on an assessment at each reporting date of the probability of achievement of the underlying milestone event. This assessment may, in certain circumstances, result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved. ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts. Expected impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. In connection with the adoption of the standard, the Company is implementing several new internal controls, including controls to monitor the probability of achievement of contingent milestone payments and the pattern of performance of certain performance obligations. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) right-of-use In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments zero-coupon In November 2016, the FASB issued ASU 2016-18, Restricted Cash 2016-18, 2016-18 2016-18 2016-18 In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting 2017-09 Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of September 30, 2017, 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. Share-Based Payment As of January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting paid-in-capital

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following table summarizes the available-for-sale Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 35,878 $ — $ (7 ) $ 35,871 Corporate notes 49,086 — (22 ) 49,064 U.S. government agency securities and U.S. Treasuries 2,499 — 2,499 Total $ 87,463 $ — $ (29 ) $ 87,434 The following table summarizes the available-for-sale Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297 The amortized cost of available-for-sale available-for-sale available-for-sale The aggregate fair value of available-for-sale

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company’s financial instruments as of September 30, 2017 and December 31, 2016 consisted primarily of cash and cash equivalents, marketable securities and accounts receivable and accounts payable. As of September 30, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of September 30, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 203,869 $ 196,377 $ 7,492 $ — Marketable securities: Commercial paper 35,871 — 35,871 — Corporate notes 49,064 — 49,064 — U.S. government agency securities and treasuries 2,499 — 2,499 — Total $ 291,303 $ 196,377 $ 94,926 $ — 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 — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ — 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 cash equivalents that the Company classifies within Level 1 of the fair value hierarchy are classified within Level 1 because they are valued using observable inputs that reflect quoted prices for identical assets in active markets. The Company measures its marketable securities at fair value on a recurring basis and classifies those instruments and some cash equivalents within Level 2 of the fair value hierarchy. The pricing services used by management utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine the fair value of marketable securities and some cash equivalents.

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Supplemental Balance Sheet Information	5. Supplemental Balance Sheet Information Accrued expenses consisted of the following: September 30, 2017 December 31, 2016 (In thousands) Employee compensation and benefits $ 4,115 $ 4,100 Research and development expenses 10,066 10,925 Professional services and other 3,366 982 Accrued expenses $ 17,547 $ 16,007

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company did not record a federal or state income tax provision or benefit for the three and nine months ended September 30, 2017 and 2016 due to the expected loss before income taxes to be incurred for the years ended December 31, 2017 and 2016, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies There have been no significant changes to the Company’s commitments and contingencies in the three and nine months ended September 30, 2017, as compared to those disclosed in Note 7, Commitments and Contingencies A $1.5 million payment obligation was incurred and paid in the nine months ended September 30, 2017 upon the achievement of a milestone under the companion diagnostic agreement with Roche Molecular Systems, Inc. (“Roche Molecular”). Lease The Company leases office and laboratory space at Technology Square in Cambridge, Massachusetts under a Lease Agreement, dated as of June 15, 2012, as amended (the “Lease”) with ARE-TECH In May 2017, the Company entered into a Third Amendment to Lease (the “Third Amendment”) with the Landlord, and a Fourth Amendment to Lease with the Landlord (the “Fourth Amendment,” and, together with the Third Amendment, the “Amendments”). The Amendments each amend the Lease. Under the Amendments, the Company extended the term of the Lease at the Company’s headquarters in Cambridge, Massachusetts to November 30, 2022, subject to the Company’s right to terminate the Lease effective as of December 31, 2018, by giving written notice to the Landlord by December 31, 2017 and paying an early termination fee. Under the Lease, the Company has agreed to pay a monthly base rent of approximately $0.2 million for the period commencing December 1, 2017 through May 31, 2018, with an increase on June 1, 2018 and annual increases December 1 of each subsequent year until December 1, 2021. Under the Lease as amended by the Amendments, the Company is responsible for aggregate minimum rent payments of $18.6 million, of which approximately $1.2 million was paid prior to September 30, 2017. The remaining future minimum rent payments from October 1, 2017 through November 30, 2022 are as follows (in thousands): October 1, 2017 – December 31, 2017 $ 696 2018 3,074 2019 3,335 2020 3,435 2021 3,538 2022 3,333 Total $ 17,411

Collaborations

Collaborations	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	8. 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 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 mid-single mid-teens 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 Amended and Restated Agreement Structure Under the amended and restated collaboration and license agreement: • Celgene retained its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat, • Celgene’s other option rights were narrowed to small molecule HMT inhibitors targeting three predefined targets (the “Option Targets”), • The exclusive licenses to HMT inhibitors targeting two of the Option Targets that Celgene may acquire were expanded to include the United States, with the exclusive license to HMT inhibitors targeting the third Option Target continuing to be for all countries other than the United States, • Celgene’s option period was extended for each of the Option Targets and Celgene’s option is exercisable at the time of the Company’s investigational new drug application (“IND”) filing for an HMT inhibitor targeting the applicable Option Target, upon the payment by Celgene at such time of a pre-specified • 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 • The Company’s research and development obligations with respect to each Option Target under the amended and restated agreement were extended for at least an additional three years, subject to Celgene exercising its option with respect to such Option Target at IND filing. Subject to the Company’s opt-out 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 co-fund Collaboration Revenue Through September 30, 2017, the Company has recognized $74.3 million of total collaboration revenue since the inception of the collaboration, including $0.5 million and $1.4 million in the three and nine months ended September 30, 2016, respectively. No revenue from the collaboration was earned in the three and nine months ended September 30, 2017. No global development co-funding co-funding GSK In January 2011, the Company entered into a collaboration and license agreement with GSK, to discover, develop and commercialize novel small molecule HMT inhibitors directed to available targets from the Company’s platform. Under the terms of the agreement, the Company granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration, the Company agreed to provide research and development services related to the licensed targets pursuant to agreed upon research plans during a research term that ended January 8, 2015. In March 2014, the Company and GSK amended certain terms of this agreement for the third licensed target, revising the license terms with respect to candidate compounds and amending the corresponding financial terms, including reallocating milestone payments and increasing royalty rates as to the third target. Subsequent to a GSK strategic portfolio prioritization, the Company received notice in October 2017 that GSK terminated the agreement with respect to the third target, effective December 31, 2017, which returns all rights to that target to the Company. Two other targets continue to be subject to the agreement and are not impacted by the termination with respect to the third target. The Company substantially completed all research obligations under this agreement by the end of the first quarter of 2015 and completed the transfer of the remaining data and materials for these programs to GSK in the second quarter of 2015. Agreement Structure Under the agreement, the Company has received and recognized as collaboration revenue a $20.0 million upfront payment, a $3.0 million payment upon the execution of the March 2014 agreement amendment, $6.0 million of fixed research funding, $9.0 million for research and development services and $31.0 million of preclinical research and development milestone payments. The preclinical and research and development milestone payments total includes a $10.0 million milestone payment earned in May 2017 related to the second target in the collaboration, upon GSK’s initiation of good laboratory practices toxicology studies. As of September 30, 2017, for the two remaining targets, the Company is eligible to receive up to $70.0 million in clinical development milestone payments, up to $197.0 million in regulatory milestone payments and up to $128.0 million in sales-based milestone payments. As a result of the termination of the agreement as it relates to the third target, no additional payments will be received related to that target. In addition, GSK is required to pay the Company royalties, at percentages from the mid-single product-by-licensed Collaboration Revenue Through September 30, 2017, the Company has earned a total of $69.0 million under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss, including $10.0 million of milestone revenue in the nine months ended September 30, 2017 and $6.0 million of milestone revenue in the nine months ended September 30, 2016. The Company recognized $10.0 million of collaboration revenue in the nine months ended September 30, 2017 related to achievement of the milestone described in the preceding paragraph. The Company did not have any deferred revenue related to this agreement as of September 30, 2017 or December 31, 2016 and any future revenues will relate to any milestone payments and royalties received under the agreement with respect to the two remaining targets, if any. Roche Molecular In December 2012, Eisai and the Company entered into an agreement with Roche Molecular under which Eisai and the Company engaged Roche Molecular to develop a companion diagnostic to identify patients who possess certain activating mutations of EZH2. In October 2013, this agreement was amended to include additional mutations in EZH2. The development costs due under the amended agreement with Roche Molecular were the responsibility of Eisai until the execution of the amended and restated collaboration and license agreement with Eisai in March 2015, at which time the Company assumed responsibility for the remaining development costs due under the agreement. In December 2015, the Company entered into a second amendment to the companion diagnostic agreement with Roche Molecular. As of September 30, 2017, the Company is responsible for the remaining development costs of $10.5 million due under the agreement. The Company expects the remaining development costs under the agreement to be incurred and paid through 2019. 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.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation Total stock-based compensation expense related to stock options, restricted stock units and the employee stock purchase plan was $2.9 million and $2.7 million for the three months ended September 30, 2017 and 2016, respectively, and $8.7 million and $7.8 million for the nine months ended September 30, 2017 and 2016, respectively. Stock-based compensation expense is classified in the consolidated statements of operations and comprehensive loss as follows: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands) Research and development $ 1,398 $ 1,390 $ 4,299 $ 4,003 General and administrative 1,497 1,327 4,374 3,840 Total $ 2,895 $ 2,717 $ 8,673 $ 7,843 Stock Options The weighted-average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $9.98 and $6.43 per option for those options granted during the three months ended September 30, 2017 and 2016, respectively, and $8.77 and $6.43 per option for those options granted during the nine months ended September 30, 2017 and 2016, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended September 30 Nine Months Ended September 30 2017 2016 2017 2016 Risk-free interest rate 1.8 % 1.1 % 1.8 % 1.2 % Expected life of options 6 years 6 years 6 years 6 years Expected volatility of underlying stock 73.4 % 76.2 % 74.3 % 78.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % The following is a summary of stock option activity for the nine months ended September 30, 2017: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2016 4,059 $ 14.32 Granted 2,213 13.37 Exercised (394 ) 6.70 Forfeited or expired (964 ) 14.25 Outstanding at September 30, 2017 4,914 $ 14.52 8.21 $ 28,561 Exercisable at September 30, 2017 1,620 $ 17.22 6.63 $ 8,063 As of September 30, 2017, there was $26.5 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.8 years. Restricted Stock Units The following is a summary of restricted stock unit activity for the nine months ended September 30, 2017: Number of Units (in thousands) Weighted Average Grant Date Fair Value per Unit Outstanding at December 31, 2016 64 $ 12.20 Granted — — Vested (20 ) 12.20 Forfeited (44 ) 12.20 Outstanding at September 30, 2017 — $ — As of September 30, 2017, there were no restricted stock units outstanding.

Loss Per Share

Loss Per Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Loss Per Share	10. Loss Per Share Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands except per share data) Net loss $ (37,597 ) $ (24,336 ) $ (98,143 ) $ (75,196 ) Weighted average shares outstanding 59,899 57,970 58,837 56,828 Basic and diluted loss per share allocable to common stockholders $ (0.63 ) $ (0.42 ) $ (1.67 ) $ (1.32 ) In September 2017, the Company issued 10,557,000 shares of common stock in a public offering. The issuance of these shares contributed to the increase in the Company’s shares outstanding as of September 30, 2017 and in the weighted average shares outstanding for the three and nine months ended September 30, 2017 when compared to the comparable prior year period, and will continue to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations. The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands) Stock options 4,914 4,015 4,914 4,015 Unvested restricted stock units 0 71 0 71 Shares issuable under employee stock purchase plan 7 18 7 18 4,921 4,104 4,921 4,104

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	11. Related Party Transactions Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 5.3% of the Company’s outstanding common stock as of September 30, 2017. Refer to Note 8, Collaborations,

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K The unaudited condensed consolidated financial statements include the accounts of Epizyme, Inc. and its wholly owned, controlled subsidiary, Epizyme Securities Corporation. All intercompany transactions and balances of subsidiaries have been eliminated in consolidation. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the results for the reported interim periods. 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 three months ended September 30, 2017 and 2016 are referred to as the third quarter of 2017 and 2016, respectively. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period.
Significant Accounting Policies	Significant Accounting Policies There have been no material changes to the Company’s significant accounting policies during the nine months ended September 30, 2017, as compared to the significant accounting policies disclosed in Note 2, Summary of Significant Accounting Policies
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue From Contracts With Customers 2014-09 Revenue Recognition 2016-10 2016-12, ASU 2014-09. 2014-09 2014-09, Revenue From Contracts with Customers The Company has not yet completed its assessment of the impact of the adoption of this standard on its consolidated financial statements, but expects that the adoption will at least impact the amount and timing of revenues recognized related to its collaboration agreement with Celgene and the impact may be material. The Company is still assessing its collaboration agreement with Celgene; however, it currently expects that (a) the number of performance obligations identified under ASC 606 will be different from the number of units of accounting identified under ASC 605 and (b) the transaction price will be different from the transaction price determined under ASC 605. In particular, the Company currently expects that the accounting treatment for Celgene’s option rights that were determined to be non-substantive The Company also expects the accounting for contingent milestone payments under its collaboration agreements to change under ASC 606. ASC 606 does not contain guidance specific to milestone payments, thereby requiring contingent milestone payments to be considered in accordance with the overall model of ASC 606 as variable consideration. Revenue from contingent milestone payments may be recognized earlier under ASC 606 than under ASC 605, based on an assessment at each reporting date of the probability of achievement of the underlying milestone event. This assessment may, in certain circumstances, result in the recognition of revenue related to a contingent milestone payment before the milestone event has been achieved. ASC 606 requires more robust disclosures than required by previous guidance, including disclosures related to disaggregation of revenue into appropriate categories, performance obligations, the judgments made in revenue recognition determinations, adjustments to revenue which relate to activities from previous quarters or years, any significant reversals of revenue, and costs to obtain or fulfill contracts. Expected impacts from the adoption of this standard could differ upon the final adoption and implementation of the standard. In connection with the adoption of the standard, the Company is implementing several new internal controls, including controls to monitor the probability of achievement of contingent milestone payments and the pattern of performance of certain performance obligations. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) right-of-use In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments zero-coupon In November 2016, the FASB issued ASU 2016-18, Restricted Cash 2016-18, 2016-18 2016-18 2016-18 In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting 2017-09
Going Concern	Going Concern At each reporting period, the Company evaluates whether there are conditions or events that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. The Company is required to make certain additional disclosures if it concludes substantial doubt exists and it is not alleviated by the Company’s plans or when its plans alleviate substantial doubt about the Company’s ability to continue as a going concern. The Company’s evaluation entails analyzing prospective operating budgets and forecasts for expectations of the Company’s cash needs, and comparing those needs to the current cash, cash equivalent and marketable security balances. After considering the Company’s current research and development plans and the timing expectations related to the progress of its programs, and after considering its existing cash, cash equivalents and marketable securities as of September 30, 2017, 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.
Share-Based Payment	Share-Based Payment As of January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting paid-in-capital

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available-for-sale Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 35,878 $ — $ (7 ) $ 35,871 Corporate notes 49,086 — (22 ) 49,064 U.S. government agency securities and U.S. Treasuries 2,499 — 2,499 Total $ 87,463 $ — $ (29 ) $ 87,434 The following table summarizes the available-for-sale Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 68,407 $ — $ (32 ) $ 68,375 Corporate notes 70,489 — (81 ) 70,408 U.S. government agency securities and U.S. Treasuries 25,507 7 — 25,514 Total $ 164,403 $ 7 $ (113 ) $ 164,297

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of September 30, 2017 and December 31, 2016, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of September 30, 2017 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 203,869 $ 196,377 $ 7,492 $ — Marketable securities: Commercial paper 35,871 — 35,871 — Corporate notes 49,064 — 49,064 — U.S. government agency securities and treasuries 2,499 — 2,499 — Total $ 291,303 $ 196,377 $ 94,926 $ — 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 — Fair Value as of December 31, 2016 Total Level 1 Level 2 Level 3 (In thousands) Corporate notes 70,408 — 70,408 — U.S. government agency securities and treasuries 25,514 — 25,514 — Total $ 227,151 $ 59,862 $ 167,289 $ —

Supplemental Balance Sheet In20

Supplemental Balance Sheet Information (Tables)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: September 30, 2017 December 31, 2016 (In thousands) Employee compensation and benefits $ 4,115 $ 4,100 Research and development expenses 10,066 10,925 Professional services and other 3,366 982 Accrued expenses $ 17,547 $ 16,007

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of Remaining Future Minimum Rent Payments	The remaining future minimum rent payments from October 1, 2017 through November 30, 2022 are as follows (in thousands): October 1, 2017 – December 31, 2017 $ 696 2018 3,074 2019 3,335 2020 3,435 2021 3,538 2022 3,333 Total $ 17,411

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
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: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands) Research and development $ 1,398 $ 1,390 $ 4,299 $ 4,003 General and administrative 1,497 1,327 4,374 3,840 Total $ 2,895 $ 2,717 $ 8,673 $ 7,843
Assumptions Used in Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended September 30 Nine Months Ended September 30 2017 2016 2017 2016 Risk-free interest rate 1.8 % 1.1 % 1.8 % 1.2 % Expected life of options 6 years 6 years 6 years 6 years Expected volatility of underlying stock 73.4 % 76.2 % 74.3 % 78.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 %
Summary of Stock Option Activity	The following is a summary of stock option activity for the nine months ended September 30, 2017: Number of Options Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (In thousands) (In years) (In thousands) Outstanding at December 31, 2016 4,059 $ 14.32 Granted 2,213 13.37 Exercised (394 ) 6.70 Forfeited or expired (964 ) 14.25 Outstanding at September 30, 2017 4,914 $ 14.52 8.21 $ 28,561 Exercisable at September 30, 2017 1,620 $ 17.22 6.63 $ 8,063
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the nine months ended September 30, 2017: Number of Units (in thousands) Weighted Average Grant Date Fair Value per Unit Outstanding at December 31, 2016 64 $ 12.20 Granted — — Vested (20 ) 12.20 Forfeited (44 ) 12.20 Outstanding at September 30, 2017 — $ —

Loss Per Share (Tables)

Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Basic and Diluted Loss per Share	Basic and diluted loss per share allocable to common stockholders are computed as follows: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands except per share data) Net loss $ (37,597 ) $ (24,336 ) $ (98,143 ) $ (75,196 ) Weighted average shares outstanding 59,899 57,970 58,837 56,828 Basic and diluted loss per share allocable to common stockholders $ (0.63 ) $ (0.42 ) $ (1.67 ) $ (1.32 )
Common Stock Equivalents Excluded from Calculation of Diluted Loss per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted loss per share allocable to common stockholders because their inclusion would have been anti-dilutive: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (In thousands) Stock options 4,914 4,015 4,914 4,015 Unvested restricted stock units 0 71 0 71 Shares issuable under employee stock purchase plan 7 18 7 18 4,921 4,104 4,921 4,104

Overview - Additional Informati

Overview - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Basis Of Presentation [Line Items]
Proceeds from sale of redeemable convertible preferred stock		$ 76,000
Cash, cash equivalents, and marketable securities	$ 307,200	307,200
Net proceeds from sale of shares after underwriting discounts and commissions, before direct and incremental costs		$ 152,920	$ 130,067
Initial public offering completion date		2013-05
Accumulated deficit	$ (451,931)	$ (451,931)		$ (353,673)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount		891,600
Non-equity funding through collaboration agreement		217,800
IPO [Member]
Basis Of Presentation [Line Items]
Sale of common stock in public offering		597,800
Proceeds from sale of redeemable convertible preferred stock		$ 152,900
Sale of common stock in public offering, shares	10,557,000
Common stock price per share	$ 15.25	$ 15.25
Net proceeds from sale of shares after underwriting discounts and commissions, before direct and incremental costs	$ 151,300

Summary of Significant Accoun25

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)
Accounting Policies [Abstract]
Change in amount of valuation allowance and deferred tax asset	$ 25.7
Amount charged to retained earnings	$ 0.1

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 87,463	$ 164,403
Available-For-Sale Securities Held, Unrealized Gains		7
Available-For-Sale Securities Held, Unrealized Losses	(29)	(113)
Available-For-Sale Securities Held, Fair Value	87,434	164,297
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	35,878	68,407
Available-For-Sale Securities Held, Unrealized Losses	(7)	(32)
Available-For-Sale Securities Held, Fair Value	35,871	68,375
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	49,086	70,489
Available-For-Sale Securities Held, Unrealized Losses	(22)	(81)
Available-For-Sale Securities Held, Fair Value	49,064	70,408
U.S. Government Agency Securities and Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	2,499	25,507
Available-For-Sale Securities Held, Unrealized Gains		7
Available-For-Sale Securities Held, Fair Value	$ 2,499	$ 25,514

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2017USD ($)
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses) recognized on sale of marketable equity securities	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 70,000,000	$ 70,000,000
Weighted average maturity period		1 month
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		$ 100,000

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 203,869	$ 62,854
Total	291,303	227,151
Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	35,871	68,375
Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	49,064	70,408
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	2,499	25,514
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	196,377	59,862
Total	196,377	59,862
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	7,492	2,992
Total	94,926	167,289
Level 2 [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	35,871	68,375
Level 2 [Member] | Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	49,064	70,408
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	$ 2,499	$ 25,514

Supplemental Balance Sheet In29

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$ 4,115	$ 4,100
Research and development expenses	10,066	10,925
Professional services and other	3,366	982
Accrued expenses	$ 17,547	$ 16,007

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Federal income tax provision or benefit	$ 0	$ 0	$ 0	$ 0
State income tax provision or benefit	$ 0	$ 0	$ 0	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Sep. 29, 2017	Sep. 30, 2017
Other Commitments [Line Items]
Monthly base rent		$ 0.2
Aggregate minimum rent payments		$ 18.6
Rent paid	$ 1.2
Initial Term [Member]
Other Commitments [Line Items]
Lease expiration date		May 31, 2018
Extended Term [Member]
Other Commitments [Line Items]
Lease expiration date		Nov. 30, 2022
Roche [Member]
Other Commitments [Line Items]
Payment obligation incurred and paid		$ 1.5

Commitments And Contingencies32

Commitments And Contingencies - Schedule of Remaining Future Minimum Rent Payments (Detail) $ in Thousands	Sep. 30, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
October 1, 2017 - December 31, 2017	$ 696
2,018	3,074
2,019	3,335
2,020	3,435
2,021	3,538
2,022	3,333
Total	$ 17,411

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	Jul. 08, 2015OptionTargets	May 31, 2017USD ($)	Mar. 31, 2014USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2013USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue					$ 6,584,000	$ 10,000,000	$ 7,529,000
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
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 received									68,000,000
Clinical development milestone achieved									25,000,000
Global development co-funding				0	100,000	100,000	100,000		7,000,000
Number of option targets | OptionTargets	3
Collaboration revenue				0	$ 500,000	0	1,400,000		74,300,000
Deferred revenue				28,800,000		28,800,000			28,800,000		28,800,000	$ 28,800,000
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
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
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 received			$ 3,000,000
Clinical development milestone payment				70,000,000		70,000,000			70,000,000		70,000,000
Additional milestone payments				197,000,000		197,000,000			197,000,000		197,000,000
Sales-based milestone payments				128,000,000		128,000,000			128,000,000		128,000,000
Collaboration revenue						10,000,000	$ 6,000,000
Deferred revenue				0		0			0		0	$ 0
Fixed research funding received										$ 6,000,000
Milestone payments received										31,000,000
Research and development services										$ 9,000,000
Preclinical and research and development milestone payments received		$ 10,000,000
Additional payments received						0
Cash and accounts receivable											69,000,000
GSK [Member] | Collaboration Revenue [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Upfront payment received								$ 20,000,000
Collaborative Arrangement [Member] | Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments				$ 10,500,000		$ 10,500,000			$ 10,500,000		$ 10,500,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options and restricted stock	$ 2,895	$ 2,717	$ 8,673	$ 7,843
Weighted-average fair value of options granted	$ 9.98	$ 6.43	$ 8.77	$ 6.43
Unrecognized compensation cost related to stock options	$ 26,500		$ 26,500
Expected weighted average remaining vesting period for recognized cost			2 years 9 months 18 days
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units outstanding	0		0		64,000

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,895	$ 2,717	$ 8,673	$ 7,843
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,398	1,390	4,299	4,003
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 1,497	$ 1,327	$ 4,374	$ 3,840

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used in Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.80%	1.10%	1.80%	1.20%
Expected life of options	6 years	6 years	6 years	6 years
Expected volatility of underlying stock	73.40%	76.20%	74.30%	78.60%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance | shares	4,059
Number of Options, Granted | shares	2,213
Number of Options, Exercised | shares	(394)
Number of Options, Forfeited or expired | shares	(964)
Number of Options, Outstanding, Ending balance | shares	4,914
Number of Options, Exercisable | shares	1,620
Weighted Average Exercise Price per Share, Outstanding, Beginning balance | $ / shares	$ 14.32
Weighted Average Exercise Price per Share, Granted | $ / shares	13.37
Weighted Average Exercise Price per Share, Exercised | $ / shares	6.70
Weighted Average Exercise Price per Share, Forfeited or expired | $ / shares	14.25
Weighted Average Exercise Price per Share, Outstanding, Ending balance | $ / shares	14.52
Weighted Average Exercise Price per Share, Exercisable | $ / shares	$ 17.22
Weighted Average Remaining Contractual Term (In Years), Outstanding	8 years 2 months 16 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	6 years 7 months 17 days
Aggregate Intrinsic Value, Outstanding | $	$ 28,561
Aggregate Intrinsic Value, Exercisable | $	$ 8,063

Stock-Based Compensation - Su38

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) - Unvested Restricted Stock Units [Member]	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Outstanding, Beginning balance | shares	64,000
Number of Units, Granted | shares	0
Number of Units, Vested | shares	(20,000)
Number of Units, Forfeited | shares	(44,000)
Number of Units, Outstanding, Ending balance | shares	0
Weighted Average Grant Date Fair Value per Unit, Beginning balance | $ / shares	$ 12.20
Weighted Average Grant Date Fair Value per Unit, Granted | $ / shares	0
Weighted Average Grant Date Fair Value per Unit, Vested | $ / shares	12.20
Weighted Average Grant Date Fair Value per Unit, Forfeited | $ / shares	12.20
Weighted Average Grant Date Fair Value per Unit, Ending balance | $ / shares	$ 0

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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net loss	$ (37,597)	$ (24,336)	$ (98,143)	$ (75,196)
Weighted average shares outstanding	59,899	57,970	58,837	56,828
Basic and diluted loss per share allocable to common stockholders	$ (0.63)	$ (0.42)	$ (1.67)	$ (1.32)

Loss Per Share - Additional Inf

Loss Per Share - Additional Information (Detail)	1 Months Ended
	Sep. 30, 2017shares
IPO [Member]
Earnings Per Share [Line Items]
Sale of common stock in public offering, shares	10,557,000

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	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,921	4,104	4,921	4,104
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,914	4,015	4,914	4,015
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	0	71	0	71
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	7	18	7	18

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - shares	Sep. 30, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Equity investments in common stock	69,237,000	58,050,000
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock	3,674,640
Equity investments in outstanding common stock percentage	5.30%