Epizyme (EPZM) 10-Q 3 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Oct. 28, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
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		58,011,368

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents	$ 66,028	$ 208,323
Marketable securities	197,315
Accounts receivable	6,200	262
Prepaid expenses and other current assets	5,460	4,478
Total current assets	275,003	213,063
Property and equipment, net	3,320	4,089
Restricted cash and other assets	663	751
Total Assets	278,986	217,903
Current Liabilities:
Accounts payable	3,387	4,653
Accrued expenses	11,385	11,335
Current portion of capital lease obligation	605	561
Current portion of deferred revenue	478	1,900
Total current liabilities	15,855	18,449
Capital lease obligation, net of current portion	270	730
Deferred revenue, net of current portion	28,809	28,809
Other long-term liabilities	256	383
Commitments and contingencies
Stockholders' Equity:
Preferred stock, $0.0001 par value; 5,000 shares authorized; 0 shares issued and outstanding
Common stock $0.0001 par value; 125,000 shares authorized; 58,002 shares and 41,786 shares issued and outstanding, respectively	6	4
Additional paid-in capital	552,542	412,989
Accumulated other comprehensive income (loss)	(95)
Accumulated deficit	(318,657)	(243,461)
Total stockholders' equity	233,796	169,532
Total Liabilities and Stockholders' Equity	$ 278,986	$ 217,903

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	58,002,000	41,786,000
Common stock, shares outstanding	58,002,000	41,786,000

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Collaboration revenue	$ 6,584	$ 358	$ 7,529	$ 2,005
Operating expenses:
Research and development	23,888	16,788	63,078	94,390
General and administrative	7,522	6,676	20,792	17,883
Total operating expenses	31,410	23,464	83,870	112,273
Loss from operations	(24,826)	(23,106)	(76,341)	(110,268)
Other income, net:
Interest income, net	469	20	1,093	57
Other income	21	21	52	61
Other income, net	490	41	1,145	118
Net loss	(24,336)	(23,065)	(75,196)	(110,150)
Other Comprehensive loss:
Unrealized loss on available for sale securities	(120)		(95)
Comprehensive loss	$ (24,456)	$ (23,065)	$ (75,291)	$ (110,150)
Loss per share allocable to common stockholders:
Basic	$ (0.42)	$ (0.56)	$ (1.32)	$ (2.81)
Diluted	$ (0.42)	$ (0.56)	$ (1.32)	$ (2.81)
Weighted average shares outstanding:
Basic	57,970	41,461	56,828	39,204
Diluted	57,970	41,461	56,828	39,204

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (75,196)	$ (110,150)
Adjustments to reconcile net loss to net cash used in operating activities:
Acquired in-process research and development		40,000
Depreciation and amortization	1,189	1,058
Stock-based compensation	7,843	7,766
Amortization of discount on investments	(52)
Loss on disposal of property and equipment		5
Changes in operating assets and liabilities:
Accounts receivable	(5,938)	1,898
Prepaid expenses and other current assets	(564)	336
Accounts payable	(1,266)	(5,328)
Accrued expenses	50	5,038
Deferred revenue	(1,422)	8,113
Restricted cash and other assets	88	(29)
Other long-term liabilities	(127)	(23)
Net cash used in operating activities	(75,395)	(51,316)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of available for sale securities	(212,126)
Maturities of available for sale securities	14,350
Acquisition of in-process research and development		(40,000)
Purchases of property and equipment	(420)	(188)
Net cash used in investing activities	(198,196)	(40,188)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment under capital lease obligation	(416)	(309)
Proceeds from public offering, net of commissions	130,067	130,712
Proceeds from stock options exercised	1,645	875
Issuance of shares under employee stock purchase plan	374	436
Payment of public offering costs	(374)	(367)
Net cash provided by financing activities	131,296	131,347
Net (decrease) increase in cash and cash equivalents	(142,295)	39,843
Cash and cash equivalents, beginning of period	208,323	190,095
Cash and cash equivalents, end of period	$ 66,028	229,938
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Equipment acquired under capital lease		$ 1,732

Overview

Overview	9 Months Ended
	Sep. 30, 2016
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 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 8, Collaborations Collaborations Through September 30, 2016, the Company has raised, including amounts receivable under collaboration agreements, an aggregate of $728.4 million to fund its operations, of which $207.8 million was non-equity funding through its collaboration agreements, $444.6 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 September 30, 2016, the Company had $263.3 million in cash, cash equivalents, and marketable securities. The Company commenced active operations in early 2008. Since its inception, the Company has generated an accumulated deficit of $318.7 million through September 30, 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	9 Months Ended
	Sep. 30, 2016
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 for the fiscal year ended December 31, 2015 (the “Annual Report”). 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, 2016 and 2015 are referred to as the third quarter of 2016 and 2015, 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 Research and Development Expenses As the clinical development plan for tazemetostat progresses, the Company expects research and development expenses and, in particular, the accounting for clinical trial accruals to be an increasingly significant accounting policy. 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. 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. There have been no other material changes or other required disclosures to the Company’s significant accounting policies during the three and nine months ended September 30, 2016, 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”) No. 2014-09, Revenue From Contracts With Customers Revenue Recognition In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern Presentation of Financial Statements—Going Concern 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

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following table summarizes the available for sale securities held at September 30, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 79,786 $ — $ (58 ) $ 79,728 Corporate notes 82,116 — (61 ) 82,055 U.S. government agency securities and U.S. Treasuries 35,507 25 — 35,532 Total $ 197,409 $ 25 $ (119 ) $ 197,315 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 on the condensed 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 September 30, 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 were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the three months and nine months ended September 30, 2016, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same period. The aggregate fair value of available for sale securities held by the Company in an unrealized loss position for less than twelve months as of September 30, 2016 was $137.6 million. The aggregate unrealized loss for those securities in an unrealized loss position for less than twelve months as of September 30, 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 September 30, 2016. The weighted average maturity of the Company’s portfolio was approximately five months at September 30, 2016.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company’s financial instruments as of September 30, 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 September 30, 2016 and December 31, 2015, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of September 30, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 44,257 $ 41,008 $ 3,249 $ — Marketable securities: Commercial paper 79,728 — 79,728 — Corporate notes 82,055 — 82,055 — U.S. government agency securities and treasuries 35,532 — 35,532 — Total $ 241,572 $ 41,008 $ 200,564 $ — 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.

Supplemental Balance Sheet Info

Supplemental Balance Sheet Information	9 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Supplemental Balance Sheet Information	5. Supplemental Balance Sheet Information Prepaid expenses and other current assets consisted of the following: September 30, 2016 December 31, 2015 (In thousands) Prepaid clinical and manufacturing costs $ 2,995 $ 2,400 Interest receivable on available for sale securities 463 — Other prepaid expenses and other receivables 2,002 2,078 Total prepaid expenses and other current assets $ 5,460 $ 4,478 Accrued expenses consisted of the following: September 30, 2016 December 31, 2015 (In thousands) Employee compensation and benefits $ 3,179 $ 3,314 Research and development expenses 7,190 6,518 Professional services and other 1,016 1,503 Accrued expenses $ 11,385 $ 11,335

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 due to the expected loss before income taxes to be incurred for the years ended December 31, 2016 and 2015, 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, 2016
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, 2016, as compared to those disclosed in Note 6, Commitments and Contingencies The Company made a $3.0 million milestone payment in the first quarter of 2016 under the second amendment to the companion diagnostic agreement with Roche Molecular Systems, Inc. (“Roche”). In May 2016, the Company entered into the second amendment to its lease arrangement with ARE-TECH Square, LLC to extend the term of the leased headquarters facility by six months, from November 30, 2017 to May 31, 2018. In addition, the second amendment extended the Company’s five year renewal option notice date by six months, from February 28, 2017 to August 31, 2017. An extension fee of $0.2 million will be due and payable on November 30, 2017 if the Company does not (i) exercise its five year option to renew the headquarter lease on November 30, 2017 or (ii) sign a new lease with its existing landlord for a location with increased square footage.

Collaborations

Collaborations	9 Months Ended
	Sep. 30, 2016
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. In July 2015, the Company entered into an amendment and restatement of its collaboration and license agreement with Celgene. 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 DOT1L, 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 other HMT targets, excluding EZH2 and targets covered by the collaboration and license agreement with GSK. Under the original agreement, Celgene’s option was exercisable during an option period that would have expired in July 2015. 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 option rights have been narrowed to 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 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 is exercisable at the time of the Company’s Investigational New Drug (“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 agreement have been extended for at least an additional three years, subject to Celgene exercising its option with respect to such Option Target at the time of the 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 time of the IND filing. 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. 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. 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 September 30, 2016. Under the amended 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 milestone 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 remains 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. The Company is also eligible to receive royalties on each of the Option Targets as specified in the amended and restated agreement. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone or royalty payments from Celgene. Collaboration Revenue Through September 30, 2016, the Company has recognized $73.9 million of total collaboration revenue since the inception of the collaboration, including $0.5 million and $0.4 million in the three months ended September 30, 2016 and 2015, respectively, and $1.4 million and $0.5 million in the nine months ended September 30, 2016 and 2015, respectively. The Company recognized total global development co-funding as a reduction to research and development expense of less than $0.1 million and $0.1 million in the three months ended September 30, 2016 and 2015, respectively, and $0.1 million and $1.0 million in the nine months ended September 30, 2016 and 2015, respectively. 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 protein arginine methyltransferase-5 (“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. Due to the varying stages of development of each licensed target, the Company is not able to determine the next milestone that might be achieved under this agreement, if any. GSK became solely responsible for development and commercialization for each licensed target in the collaboration when the research term ended on January 8, 2015. Collaboration Revenue Through September 30, 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 of milestone revenue in the three and nine months ended September 30, 2016 and $0 and $1.5 million in the three and nine months ended September 30, 2015, respectively. The Company does not have any remaining deferred revenue related to this agreement 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. Additionally, as part of the research collaboration, the Company provided research and development services related to the licensed compounds through December 31, 2014. 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 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 will evaluate 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 will evaluate this combination regimen for the treatment of patients with relapsed or refractory DLBCL. Under the agreement, each company will supply its respective anti-cancer agent to support the trial and share 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. Under the agreement with Roche, Roche is obligated to use commercially reasonable efforts to develop and to make commercially available the companion diagnostic. Roche has exclusive rights to commercialize the companion diagnostic. The agreement with Roche will expire when the Company is no longer developing or commercializing tazemetostat. The Company may terminate the agreement by giving Roche 90 days’ written notice if the Company discontinues development and commercialization of tazemetostat or determines, in conjunction with Roche, that the companion diagnostic is not needed for use with tazemetostat. Either the Company or Roche 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 may become entitled to specified termination fees.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2016
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 $2.7 million and $2.9 million for the three months ended September 30, 2016 and 2015, respectively, and $7.8 million and $7.8 million for the nine months ended September 30, 2016 and 2015, 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, 2016 2015 2016 2015 (In thousands) Research and development $ 1,390 $ 1,214 $ 4,003 $ 4,009 General and administrative 1,327 1,642 3,840 3,757 Total $ 2,717 $ 2,856 $ 7,843 $ 7,766 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 $6.43 and $15.78 per option for those options granted during the three months ended September 30, 2016 and 2015, respectively, and $6.43 and $15.05 per option for those options granted during the nine months ended September 30, 2016 and 2015, respectively. Key assumptions used to apply this pricing model were as follows: Three Months Ended September 30 Nine Months Ended September 30 2016 2015 2016 2015 Risk-free interest rate 1.1 % 1.6 % 1.2 % 1.6 % Expected life of options 6 years 6 years 6 years 6 years Expected volatility of underlying stock 76.2 % 82.9 % 78.6 % 83.7 % 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, 2016: 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, 2015 3,100 $ 15.20 Granted 2,107 9.50 Exercised (773 ) 2.01 Forfeited or expired (419 ) 18.31 Outstanding at September 30, 2016 4,015 $ 14.42 8.6 $ 3,808 Exercisable at September 30, 2016 1,040 $ 18.21 7.1 $ 2,327 As of September 30, 2016, there was $21.3 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 3.0 years. Restricted Stock Units The following is a summary of restricted stock unit activity for the nine months ended September 30, 2016: Number of Units Weighted Average Grant Date Fair Value per Unit (in thousands) Outstanding at December 31, 2015 37 $ 18.49 Granted 81 9.29 Vested (47 ) 12.20 Outstanding at September 30, 2016 71 $ 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 September 30, 2016 there was $0.7 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 2.4 years.

Loss Per Share

Loss Per Share	9 Months Ended
	Sep. 30, 2016
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, 2016 2015 2016 2015 (In thousands except per share data) Net loss $ (24,336 ) $ (23,065 ) $ (75,196 ) $ (110,150 ) Weighted average shares outstanding 57,970 41,461 56,828 39,204 Basic and diluted loss per share allocable to common stockholders $ (0.42 ) $ (0.56 ) $ (1.32 ) $ (2.81 ) 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 September 30, 2016 and in the weighted average shares outstanding for the three and nine months ended September 30, 2016 when compared to the comparable prior year periods, and is expected to continue to impact the year-over-year comparability of the Company’s (loss) earnings per share calculations through 2016. 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, 2016 2015 2016 2015 (In thousands) Stock options 4,015 3,119 4,015 3,119 Unvested restricted stock units 71 37 71 37 Shares issuable under employee stock purchase plan 18 2 18 2 4,104 3,158 4,104 3,158

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2016
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 6.3% of the Company’s outstanding common stock as of September 30, 2016. Refer to Note 8, Collaborations,

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2016
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 for the fiscal year ended December 31, 2015 (the “Annual Report”). 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, 2016 and 2015 are referred to as the third quarter of 2016 and 2015, 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
Research and Development Expenses	Research and Development Expenses As the clinical development plan for tazemetostat progresses, the Company expects research and development expenses and, in particular, the accounting for clinical trial accruals to be an increasingly significant accounting policy. 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.
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. There have been no other material changes or other required disclosures to the Company’s significant accounting policies during the three and nine months ended September 30, 2016, 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”) No. 2014-09, Revenue From Contracts With Customers Revenue Recognition In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties About an Entity’s Ability to Continue as a Going Concern Presentation of Financial Statements—Going Concern 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

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities Held	The following table summarizes the available for sale securities held at September 30, 2016 (in thousands): Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Commercial paper $ 79,786 $ — $ (58 ) $ 79,728 Corporate notes 82,116 — (61 ) 82,055 U.S. government agency securities and U.S. Treasuries 35,507 25 — 35,532 Total $ 197,409 $ 25 $ (119 ) $ 197,315

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Summary of Company's Financial Assets Recognized at Fair Value	As of September 30, 2016 and December 31, 2015, the Company’s financial assets recognized at fair value consisted of the following: Fair Value as of September 30, 2016 Total Level 1 Level 2 Level 3 (In thousands) Cash equivalents $ 44,257 $ 41,008 $ 3,249 $ — Marketable securities: Commercial paper 79,728 — 79,728 — Corporate notes 82,055 — 82,055 — U.S. government agency securities and treasuries 35,532 — 35,532 — Total $ 241,572 $ 41,008 $ 200,564 $ — 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 $ — $ —

Supplemental Balance Sheet In20

Supplemental Balance Sheet Information (Tables)	9 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: September 30, 2016 December 31, 2015 (In thousands) Prepaid clinical and manufacturing costs $ 2,995 $ 2,400 Interest receivable on available for sale securities 463 — Other prepaid expenses and other receivables 2,002 2,078 Total prepaid expenses and other current assets $ 5,460 $ 4,478
Schedule of Accrued Expenses	Accrued expenses consisted of the following: September 30, 2016 December 31, 2015 (In thousands) Employee compensation and benefits $ 3,179 $ 3,314 Research and development expenses 7,190 6,518 Professional services and other 1,016 1,503 Accrued expenses $ 11,385 $ 11,335

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 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: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 (In thousands) Research and development $ 1,390 $ 1,214 $ 4,003 $ 4,009 General and administrative 1,327 1,642 3,840 3,757 Total $ 2,717 $ 2,856 $ 7,843 $ 7,766
Assumptions Used to Applying Pricing Model	Key assumptions used to apply this pricing model were as follows: Three Months Ended September 30 Nine Months Ended September 30 2016 2015 2016 2015 Risk-free interest rate 1.1 % 1.6 % 1.2 % 1.6 % Expected life of options 6 years 6 years 6 years 6 years Expected volatility of underlying stock 76.2 % 82.9 % 78.6 % 83.7 % 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, 2016: 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, 2015 3,100 $ 15.20 Granted 2,107 9.50 Exercised (773 ) 2.01 Forfeited or expired (419 ) 18.31 Outstanding at September 30, 2016 4,015 $ 14.42 8.6 $ 3,808 Exercisable at September 30, 2016 1,040 $ 18.21 7.1 $ 2,327
Summary of Restricted Stock Activity	The following is a summary of restricted stock unit activity for the nine months ended September 30, 2016: Number of Units Weighted Average Grant Date Fair Value per Unit (in thousands) Outstanding at December 31, 2015 37 $ 18.49 Granted 81 9.29 Vested (47 ) 12.20 Outstanding at September 30, 2016 71 $ 12.20

Loss Per Share (Tables)

Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 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: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 (In thousands except per share data) Net loss $ (24,336 ) $ (23,065 ) $ (75,196 ) $ (110,150 ) Weighted average shares outstanding 57,970 41,461 56,828 39,204 Basic and diluted loss per share allocable to common stockholders $ (0.42 ) $ (0.56 ) $ (1.32 ) $ (2.81 )
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, 2016 2015 2016 2015 (In thousands) Stock options 4,015 3,119 4,015 3,119 Unvested restricted stock units 71 37 71 37 Shares issuable under employee stock purchase plan 18 2 18 2 4,104 3,158 4,104 3,158

Overview - Additional Informati

Overview - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Basis Of Presentation [Line Items]
Proceeds from sale of redeemable convertible preferred stock	$ 76,000
Initial public offering completion date	2013-05
Cash, cash equivalents, and marketable securities	$ 263,300
Accumulated deficit	(318,657)	$ (243,461)
Collaborative Arrangement [Member]
Basis Of Presentation [Line Items]
Aggregate fund, amount	728,400
Non-equity funding through collaboration agreement	207,800
IPO [Member]
Basis Of Presentation [Line Items]
Sale of common stock in public offering	$ 444,600

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($)	Sep. 30, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	$ 197,409,000
Available-For-Sale Securities Held, Unrealized Gains	25,000
Available-For-Sale Securities Held, Unrealized Losses	(119,000)
Available-For-Sale Securities Held, Fair Value	197,315,000	$ 0
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	79,786,000
Available-For-Sale Securities Held, Unrealized Losses	(58,000)
Available-For-Sale Securities Held, Fair Value	79,728,000
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	82,116,000
Available-For-Sale Securities Held, Unrealized Losses	(61,000)
Available-For-Sale Securities Held, Fair Value	82,055,000
U.S. Government Agency Securities and Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Held, Amortized Cost	35,507,000
Available-For-Sale Securities Held, Unrealized Gains	25,000
Available-For-Sale Securities Held, Fair Value	$ 35,532,000

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Mar. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-For-Sale Securities Held, Fair Value	$ 197,315,000	$ 197,315,000	$ 0
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	$ 137,600,000	137,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		$ 100,000
Weighted average maturity period		5 months

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 44,257	$ 197,023
Total	241,572	197,023
Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	79,728
Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	82,055
U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	35,532
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	41,008	197,023
Total	41,008	$ 197,023
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	3,249
Total	200,564
Level 2 [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	79,728
Level 2 [Member] | Corporate Notes [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	82,055
Level 2 [Member] | U.S. Government Agency Securities and Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities	$ 35,532

Supplemental Balance Sheet In27

Supplemental Balance Sheet Information - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid clinical and manufacturing costs	$ 2,995	$ 2,400
Interest receivable on available for sale securities	463
Other prepaid expenses and other receivables	2,002	2,078
Total prepaid expenses and other current assets	$ 5,460	$ 4,478

Supplemental Balance Sheet In28

Supplemental Balance Sheet Information - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$ 3,179	$ 3,314
Research and development expenses	7,190	6,518
Professional services and other	1,016	1,503
Accrued expenses	$ 11,385	$ 11,335

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
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	1 Months Ended	3 Months Ended
	May 31, 2016	Mar. 31, 2016
ARE-TECH Square, LLC [Member]
Other Commitments [Line Items]
Extension period of lease	5 years
Extension fee payable if lease not renewed	$ 0.2
Roche [Member]
Other Commitments [Line Items]
Milestone amount paid		$ 3

Collaborations - Additional Inf

Collaborations - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended		36 Months Ended	54 Months Ended	69 Months Ended
	Jul. 31, 2015OptionTargets	Mar. 31, 2015USD ($)	Mar. 31, 2014USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2013USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2016USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue				$ 6,584,000	$ 358,000	$ 7,529,000	$ 2,005,000
Upfront payment made							40,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Notice period in days						90 days
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]
Number of option targets | OptionTargets	3
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									7,000,000
Collaboration revenue				500,000	400,000	1,400,000	500,000		73,900,000
Celgene [Member] | Minimum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] | Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Global development co-funding				100,000	100,000	100,000	1,000,000
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] | 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]
Development milestone and license payments associated with the Option Targets						75,000,000
Additional milestone payments				365,000,000		365,000,000			365,000,000	365,000,000
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
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
Clinical development milestone payment				103,000,000		103,000,000			103,000,000	103,000,000
Collaboration revenue				6,000,000	$ 0	6,000,000	$ 1,500,000
Fixed research funding received										6,000,000
Milestone payments received										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
Cash and accounts receivable										59,000,000
Deferred revenue				0		0			0	0
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
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

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options and restricted stock		$ 2,717	$ 2,856	$ 7,843	$ 7,766
Weighted-average fair value of options granted		$ 6.43	$ 15.78	$ 6.43	$ 15.05
Unrecognized compensation cost related to stock options		$ 21,300		$ 21,300
Expected weighted average period for recognition of compensation cost				3 years
Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected weighted average period for recognition of compensation cost				2 years 4 months 24 days
Number of Shares, Granted				81,000
Weighted Average Grant Date Fair Value per Share, Granted				$ 9.29
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.
Unrecognized compensation cost related to restricted stock units		$ 700		$ 700
Unvested Restricted Stock Units [Member] | February 9, 2016 Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted	80,732
Weighted Average Grant Date Fair Value per Share, Granted	$ 9.29
Vesting period				36 months

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,717	$ 2,856	$ 7,843	$ 7,766
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,390	1,214	4,003	4,009
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 1,327	$ 1,642	$ 3,840	$ 3,757

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used to Applying Pricing Model (Detail) - Employee Stock Option [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.10%	1.60%	1.20%	1.60%
Expected life of options	6 years	6 years	6 years	6 years
Expected volatility of underlying stock	76.20%	82.90%	78.60%	83.70%
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, $ in Thousands	9 Months Ended
	Sep. 30, 2016USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance | shares	3,100
Number of Options, Granted | shares	2,107
Number of Options, Exercised | shares	(773)
Number of Options, Forfeited or expired | shares	(419)
Number of Options, Outstanding, Ending balance | shares	4,015
Number of Options, Exercisable | shares	1,040
Weighted Average Exercise Price per Share, Outstanding, Beginning balance | $ / shares	$ 15.20
Weighted Average Exercise Price per Share, Granted | $ / shares	9.50
Weighted Average Exercise Price per Share, Exercised | $ / shares	2.01
Weighted Average Exercise Price per Share, Forfeited or expired | $ / shares	18.31
Weighted Average Exercise Price per Share, Outstanding, Ending balance | $ / shares	14.42
Weighted Average Exercise Price per Share, Exercisable | $ / shares	$ 18.21
Weighted Average Remaining Contractual Term (In Years), Outstanding	8 years 7 months 6 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	7 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding | $	$ 3,808
Aggregate Intrinsic Value, Exercisable | $	$ 2,327

Stock-Based Compensation - Su36

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) - Unvested Restricted Stock Units [Member] shares in Thousands	9 Months Ended
	Sep. 30, 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	(47)
Number of Units, Outstanding, Ending balance | shares	71
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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net loss	$ (24,336)	$ (23,065)	$ (75,196)	$ (110,150)
Weighted average shares outstanding	57,970	41,461	56,828	39,204
Basic and diluted loss per share allocable to common stockholders	$ (0.42)	$ (0.56)	$ (1.32)	$ (2.81)

Loss per Share - Additional Inf

Loss per Share - Additional Information (Detail)	1 Months Ended
	Jan. 31, 2016shares
Earnings Per Share [Abstract]
Sale of common stock in public offering, shares	15,333,334

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	4,104	3,158	4,104	3,158
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	4,015	3,119	4,015	3,119
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	71	37	71	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	18	2	18	2

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - shares	Sep. 30, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Equity investments in common stock	58,002,000	41,786,000
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock	3,674,640
Equity investments in outstanding common stock percentage	6.30%