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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2015	Oct. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Trading Symbol	EPZM
Entity Registrant Name	EPIZYME, INC.
Entity Central Index Key	0001571498
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		41,703,345

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Current Assets:
Cash and cash equivalents	$ 229,938	$ 190,095
Accounts receivable	177	2,075
Prepaid expenses and other current assets	2,504	2,840
Total current assets	232,619	195,010
Property and equipment, net	4,477	3,620
Restricted cash and other assets	602	573
Total Assets	237,698	199,203
Current Liabilities:
Accounts payable	2,972	8,300
Accrued expenses	12,081	7,043
Current portion of capital lease obligation	548
Current portion of deferred revenue	1,900	1,702
Total current liabilities	17,501	17,045
Capital lease obligation, net of current portion	875
Deferred revenue, net of current portion	29,364	21,449
Other long-term liabilities	404	427
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; 41,698 shares and 34,426 shares issued and outstanding, respectively	4	3
Additional paid-in capital	410,785	271,364
Accumulated deficit	(221,235)	(111,085)
Total stockholders' equity	189,554	160,282
Total Liabilities and Stockholders' Equity	$ 237,698	$ 199,203

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2015	Dec. 31, 2014
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	41,698,000	34,426,000
Common stock, shares outstanding	41,698,000	34,426,000

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Collaboration revenue	$ 358	$ 8,177	$ 2,005	$ 31,062
Operating expenses:
Research and development	16,788	22,244	94,390	55,090
General and administrative	6,676	5,669	17,883	15,931
Total operating expenses	23,464	27,913	112,273	71,021
Loss from operations	(23,106)	(19,736)	(110,268)	(39,959)
Other income, net:
Interest income, net	20	24	57	66
Other income	21	17	61	41
Other income, net	41	41	118	107
Loss before income taxes	(23,065)	(19,695)	(110,150)	(39,852)
Income tax expense		5		118
Net loss	$ (23,065)	$ (19,700)	$ (110,150)	$ (39,970)
Loss per share allocable to common stockholders:
Basic and Diluted	$ (0.56)	$ (0.58)	$ (2.81)	$ (1.23)
Weighted average shares outstanding:
Basic and Diluted	41,461	33,676	39,204	32,607
Comprehensive loss	$ (23,065)	$ (19,700)	$ (110,150)	$ (39,970)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (110,150)	$ (39,970)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Acquired in-process research and development	40,000
Depreciation and amortization	1,058	548
Stock-based compensation	7,766	4,678
Loss on disposal of property and equipment	5
Changes in operating assets and liabilities:
Accounts receivable	1,898	31,558
Prepaid expenses and other current assets	336	(852)
Accounts payable	(5,328)	1,107
Accrued expenses	5,038	2,020
Deferred revenue	8,113	(13,722)
Restricted cash and other assets	(29)	226
Other long-term liabilities	(23)	9
Net cash (used in) provided by operating activities	(51,316)	(14,398)
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of in-process research and development	(40,000)
Purchases of property and equipment	(188)	(1,230)
Net cash used in investing activities	(40,188)	(1,230)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment under capital lease obligation	(309)
Proceeds from public offering, net of commissions	130,712	101,283
Proceeds from stock options exercised	875	2,348
Excess tax benefit from stock option plan		28
Issuance of shares under employee stock purchase plan	436	454
Payment of public offering costs	(367)	(649)
Proceeds from reimbursement of public offering costs		269
Net cash provided by financing activities	131,347	103,733
Net increase in cash and cash equivalents	39,843	88,105
Cash and cash equivalents, beginning of period	190,095	123,564
Cash and cash equivalents, end of period	229,938	$ 211,669
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Equipment acquired under capital lease	$ 1,732

Overview and Basis of Presentation

9 Months Ended

Sep. 30, 2015

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Overview and Basis of Presentation

1.	Overview and Basis of Presentation

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 cancer patients. The Company has built a proprietary product platform that it uses to create small molecule inhibitors of a 96-member class of enzymes known as histone methyltransferases (HMTs). Genetic alterations can result in changes to the activity of HMTs, making them oncogenic. The Company’s therapeutic strategy is to inhibit oncogenic HMTs to treat the underlying causes of the associated cancers.

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, 2014 (the “Annual Report”).

The unaudited condensed consolidated financial statements include the accounts of Epizyme and its subsidiary. 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 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, 2015 and 2014 are referred to as the third quarter of 2015 and 2014, 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.

In March 2015, the Company conducted a public offering of its common stock, selling 6,000,000 shares at a price of $20.75 per share. The Company received net proceeds before expenses from the sale of these 6,000,000 shares of $117.0 million after deducting underwriting discounts and commissions paid by the Company. In April 2015, the Company issued and sold an additional 701,448 shares in connection with the March 2015 public offering at a price of $20.75 per share pursuant to the underwriters’ option to purchase additional shares that the Company granted in connection with such public offering. The Company received net proceeds before expenses from the sale of these 701,448 shares of $13.7 million after deducting underwriting discounts and commissions paid by the Company.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2015

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

In the nine months ended September 30, 2015, the Company updated its accounting policy regarding property and equipment as a result of property and equipment acquired pursuant to a capital lease.

Property and Equipment

The Company records property and equipment at cost. Property and equipment acquired under a capital lease is recorded at the lesser of the present value of the minimum lease payments under the capital lease or the fair value of the leased property at lease inception.

The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:


Asset Category		Useful Lives
Laboratory equipment		5 - 20 years
Office furniture and equipment		3 - 10 years or term of respective lease, if shorter
Leasehold improvements		3 - 10 years or term of respective lease, if shorter

Amortization of capital lease assets is included in depreciation expense. The Company capitalizes expenditures for new property and equipment and improvements to existing facilities and charges the cost of maintenance to expense. The Company eliminates the cost of property retired or otherwise disposed of, along with the corresponding accumulated depreciation, from the related accounts, and the resulting gain or loss is reflected in the results of operations.

Additionally, the Company updated its accounting policies as a result of the amended and restated collaboration and license agreement the Company executed with Eisai Co., Ltd. (“Eisai”), pursuant to which the Company recorded the reacquisition of worldwide rights, excluding Japan, to its EZH2 program, including tazemetostat (also known as EPZ-6438), as an acquisition of in-process research and development.

Acquired In-Process Research and Development

The Company records upfront payments that relate to the acquisition of a development-stage product candidate as research and development expense in the period in which they are incurred, provided that the acquired development-stage product candidate did not also include processes or activities that would constitute a business, the product candidate has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use.

There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Annual Report.

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. ASU 2014-09 amends Accounting Standards Codification (“ASC”) 605, Revenue Recognition, by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. ASU 2014-09 was originally pronounced to become effective for the Company for interim and annual periods beginning after December 15, 2016. In July 2015, the FASB approved a one-year deferral of the effective date of ASU 2014-09. This ASU will now be effective for annual and interim periods beginning on or after December 15, 2017. Early adoption is permitted, however not before the original effective date of annual periods beginning after December 15, 2016. The Company is evaluating the impact that this ASU may have on its consolidated financial statements.

In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern. ASU 2014-15 amends ASC 205-40, Presentation of Financial Statements—Going Concern, by providing guidance on determining when and how reporting entities must disclose going-concern uncertainties in their financial statements, including requiring management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date of issuance of the entity’s financial statements and providing certain disclosures if there is substantial doubt about the entity’s ability to continue as a going concern. ASU 2014-15 will be effective for the Company for annual periods ending after December 15, 2016 and interim periods within annual periods beginning after December 15, 2016. The Company is still evaluating the impact of this ASU on its consolidated financial statements; however, it is disclosure-only in nature.

In April 2015, the FASB issued ASU No. 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement. ASU 2015-05 amends ASC 350-40, Internal-Use Software, by providing customers with guidance on determining whether a cloud computing arrangement contains a software license that should be accounted for as internal-use software. ASU 2015-05 will be effective for the Company for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2015. The Company is evaluating the impact that this ASU may have on its consolidated financial statements, if any.

In June 2015, the FASB issued ASU No. 2015-10, Technical Corrections and Improvements. ASU 2015-10 covers a wide range of Topics in the ASC. The amendments in this ASU represent changes to clarify the ASC, correct unintended application of guidance, or make minor improvements to the ASC that are not expected to have a significant effect on current accounting practice or create a significant administrative cost to most entities. Additionally, some of the amendments will make the ASC easier to understand and easier to apply by eliminating inconsistencies, providing needed clarifications, and improving the presentation of guidance in the ASC. The amendments in ASU 2015-10 that require transition guidance are effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2015. All other amendments will be effective upon the issuance of this ASU. The Company does not anticipate that the adoption of this standard will have a material impact on its consolidated financial statements and footnote disclosures.

Fair Value Measurements

9 Months Ended

Sep. 30, 2015

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The Company classifies fair value based measurements using a three-level hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows: Level 1, quoted market prices in active markets for identical assets or liabilities; Level 2, observable inputs other than quoted market prices included in Level 1 such as quoted market prices for markets that are not active or other inputs that are observable or can be corroborated by observable market data; and Level 3, unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities, including estimates and assumptions developed by the Company, reflective of those that a market participant would use, as inputs to certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs.

The Company’s financial instruments as of September 30, 2015 and December 31, 2014 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of September 30, 2015 and December 31, 2014, the Company’s financial assets recognized at fair value consisted of the following:


	Fair Value as of September 30, 2015
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	196,975	$	196,975	$	—	$	—

Total	$	196,975	$	196,975	$	—	$	—


	Fair Value as of December 31, 2014
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	184,257	$	184,257	$	—	$	—

Total	$	184,257	$	184,257	$	—	$	—

Accrued Expenses

9 Months Ended

Sep. 30, 2015

Payables and Accruals [Abstract]

Accrued Expenses

4.	Accrued Expenses

Accrued expenses consisted of the following:


	September 30, 2015		December 31, 2014
	(In thousands)
Employee compensation and benefits	$	2,780	$	2,623
Research and development and professional expenses		9,301		4,420

Accrued expenses	$	12,081	$	7,043

Income Taxes

9 Months Ended

Sep. 30, 2015

Income Tax Disclosure [Abstract]

Income Taxes

5.	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, 2015 due to the expected loss before income taxes to be incurred for the year ending December 31, 2015, as well as the Company’s continued maintenance of a full valuation allowance against its net deferred tax assets.

The Company recorded less than $0.1 million and $0.1 million of income tax expense in the three and nine months ended September 30, 2014, respectively, due to provision-to-return adjustments identified related to the year ended December 31, 2013.

Commitments and Contingencies

9 Months Ended

Sep. 30, 2015

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

6.	Commitments and Contingencies

Commitments

In the first quarter of 2015, the Company acquired computer equipment pursuant to a capital lease. Future minimum equipment lease payments under this capital lease, net of imputed interest, as of September 30, 2015 are as follows:


	(In thousands)
Future minimum lease payments in year ending December 31:
2015	$	166
2016		665
2017		665
2018		112

Total future minimum lease payments		1,608
Less: Amount representing imputed interest on equipment lease		(185	)

Capital lease obligation	$	1,423

The Company also entered into an agreement in June 2015 to lease approximately 4,000 square feet of office space in Durham, North Carolina through July 2017. Total future minimum lease payments under this office lease agreement are approximately $0.2 million.

In connection with the amended and restated collaboration and license agreement that the Company executed with Eisai in March 2015, the Company and Eisai entered into an amended and restated letter agreement related to their December 2012 companion diagnostic agreement with Roche Molecular Systems (“Roche”). Upon the execution of the amended and restated letter agreement with Eisai, the Company assumed responsibility for up to $15.5 million of the then remaining development costs under the agreement with Roche. Eisai continues to be responsible for up to $1.0 million of the remaining Japan-specific development costs under the agreement with Roche.

Contingencies

In connection with the execution of the amended and restated collaboration and license agreement with Eisai, the Company agreed to pay Eisai up to a total of $20.0 million upon the achievement of specified clinical development milestones and up to a total of $50.0 million upon the achievement of specified regulatory milestones. In addition, the Company agreed to pay Eisai royalties at a percentage in the mid-teens on worldwide net sales of any EZH2 product, excluding net sales in Japan.

Collaborations

9 Months Ended

Sep. 30, 2015

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Collaborations

7.	Collaborations

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 (the “original agreement”). Additionally, as part of the research collaboration, the Company provided research and development services related to the licensed compounds through December 31, 2014.

The Company recognized $1.4 million and $5.0 million of collaboration revenue in the three and nine months ended September 30, 2014, respectively, under the original agreement. As of December 31, 2014, the Company had completed its performance obligations under the original agreement. Accordingly, the Company had no remaining deferred revenue as of December 31, 2014 related to 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 collaboration and license agreement, the Company is responsible for global development, manufacturing and commercialization outside of Japan of tazemetostat and any other EZH2 product candidates, with Eisai retaining development and commercialization rights in Japan, as well as a right to elect to manufacture tazemetostat and any other EZH2 product candidates in Japan.

Under the original agreement, Eisai was solely responsible for funding all research, development and commercialization costs for EZH2 compounds. Under the amended and restated collaboration and license agreement, the Company is solely responsible for funding global development, manufacturing and commercialization costs for EZH2 compounds outside of Japan, including up to $15.5 million of the then 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 collaboration and license 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, has not achieved regulatory approval for marketing and, absent obtaining such approval, has 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 condensed 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.

Celgene

In April 2012, the Company entered into a collaboration and license agreement with Celgene Corporation and Celgene International Sàrl, an affiliate of Celgene Corporation (Celgene Corporation and its affiliated entities are collectively referred to as “Celgene”). On July 8, 2015, the Company entered into an amendment and restatement of the collaboration and license agreement with Celgene.

Original Agreement Structure

Under the original agreement, the Company granted Celgene an exclusive license, for all countries other than the United States, to small molecule HMT inhibitors targeting the DOT1L HMT, including pinometostat, and an option, on a target-by-target basis, to exclusively license, for all countries other than the United States, rights to small molecule HMT inhibitors targeting any HMT targets, other than the EZH2 HMT including tazemetostat and targets covered by the Company’s collaboration and license agreement dated January 8, 2011 with GlaxoSmithKline (“GSK”). Under the original agreement, Celgene’s option was exercisable during an option period that would have expired on July 9, 2015.

Under the original agreement, the Company received a $65.0 million upfront payment and $25.0 million from the sale of its series C redeemable convertible preferred stock to an affiliate of Celgene, of which $3.0 million was considered a premium and included as collaboration arrangement consideration for a total upfront payment of $68.0 million. In addition, the Company has received a $25.0 million clinical development milestone payment and $6.8 million of global development co-funding through September 30, 2015. The Company was also eligible to receive $35.0 million in an additional clinical development milestone payment and up to $100.0 million in regulatory milestone payments related to DOT1L as well as up to $65.0 million in payments, including a combination of clinical development milestone payments and an option exercise fee for each available target to which Celgene had the right to exercise its option during an initial option period that would have ended in July 2015 (each a “selected target”), and up to $100.0 million in regulatory milestone payments for each selected target. As to DOT1L and each selected target, the Company retained all product rights in the United States and was eligible to receive royalties for each target at defined percentages ranging from the mid-single digits to the mid-teens on net product sales outside of the United States subject to reduction in specified circumstances.

The Company is 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 will equally co-fund global development and each party will solely fund territory-specific development costs for its territory.

Amended and Restated Agreement Structure

Under the amended and restated collaboration and license agreement:

•

Celgene retains its exclusive license to small molecule HMT inhibitors targeting DOT1L, including pinometostat,

•

Celgene’s other option rights have been narrowed to small molecule HMT inhibitors targeting three predefined targets (the “Option Targets”),

•

The exclusive licenses to HMT inhibitors targeting two of the Option Targets that Celgene may acquire have been expanded to include the United States, with the exclusive license to HMT inhibitors targeting the third Option Target continuing to be for all countries other than the United States,

•

Celgene’s option period has been extended for each of the Option Targets and Celgene’s option is exercisable at the time of the Company’s investigational new drug application (“IND”) filing for an HMT inhibitor targeting the applicable Option Target, upon the payment by Celgene at such time of a pre-specified development milestone-based license payment,

•

Celgene’s license may be maintained beyond the end of Phase 1 clinical development for each of the Option Targets, upon payment by Celgene at such time of a pre-specified development milestone-based license payment, and

•

The Company’s research and development obligations with respect to each Option Target under the amended and restated agreement have been extended for at least an additional three years, subject to Celgene exercising its option with respect to such Option Target at IND filing. Subject to the Company’s opt-out rights, the Company’s research and development obligations have been expanded to include the completion of a Phase 1 clinical trial as to each Option Target following Celgene’s exercise of its option at IND filing.

Under the amended and restated agreement, the Company received a $10.0 million upfront payment in exchange for the Company’s extension of Celgene’s option rights to the Option Targets and the Company’s research and development obligations. In addition, the Company is eligible to earn an aggregate of up to $75.0 million in development milestones and license payments, up to $365.0 million in regulatory milestone payments and up to $170.0 million in sales milestone payments related to the three Option Targets. The Company is also eligible to receive royalties on each of the Option Targets as specified in the amended and restated agreement.

The amended and restated agreement eliminated the right of first negotiation that the Company had granted to Celgene under the original agreement with respect to business combination transactions that the Company may desire to pursue with third parties.

The Company is primarily responsible for the research strategy under the collaboration. During each applicable option period the Company is required to use commercially reasonable efforts to carry out a mutually agreed-upon research plan for each Option Target. Subject to the Company’s opt-out right, for the DOT1L target and each of the Option Targets, the Company is required to conduct and solely fund development costs of the Phase 1 clinical trials for HMT inhibitors directed to such targets, including for pinometostat. After the completion of Phase 1 development, as to DOT1L and the Option Target for which the Company retains U.S. rights, Celgene and the Company will equally co-fund global development and each party will solely fund territory-specific development costs for its respective territory; and, as to the other two Option Targets, after the completion of Phase 1 development, Celgene will solely fund all development costs on a worldwide basis.

Accounting Considerations of the Amended and Restated Agreement

The Company determined that the amended and restated agreement represents a modification of the original agreement. Accordingly, the Company determined that the remaining deferred revenue of $21.6 million attributable to the original agreement should be combined with the $10 million upfront payment received from Celgene upon execution of the amended and restated agreement. This combined amount, along with any development milestone-based license fees associated with those Option Targets where Celgene’s option rights are determined to be non-substantive, will be recognized on a prospective basis after (i) identifying the deliverables and units of accounting included in the amended and restated agreement, (ii) allocating the total arrangement fee among the units of accounting and (iii) determining the revenue recognition for each unit of accounting.

The Company evaluated the deliverables in the amended and restated agreement and determined that the deliverables included its obligation to provide research and development services for DOT1L and the three Option Targets. In addition, the Company concluded that Celgene’s option rights are not substantive and the licenses to HMT inhibitors targeting each Option Target were therefore considered to be deliverables. As a result, the Company has determined the deliverables in the amended and restated arrangement to be:

•

Research and development services for DOT1L and the three Option Targets

•

Licenses to HMT inhibitors targeting the Option Targets

The Company evaluated the nature of the deliverables to determine the units of accounting. At a program level (that is, for DOT1L and each Option Target), the Company believes there is standalone value. Therefore, the identified deliverables were evaluated on a program by program basis to determine the units of accounting.

The Company determined the research services related to DOT1L have stand-alone value as the services are delivered and therefore represent a single unit of accounting. There are no other deliverables related to the DOT1L program. For two of the Option Targets, the Company concluded the licenses and related research services should be accounted for as a combined unit of accounting up until an IND filing. Prior to an IND filing, the delivered research services did not have standalone value from the remaining undelivered elements because Celgene could not resell the research services and no other vendor has the specialized skills and know-how related to HMT inhibitors necessary to provide the research services. After an IND filing, for these two Option Targets, the Company concluded the delivered pre-IND research services and licenses, once delivered, would have standalone value from the remaining research services because Celgene, or other market participants, would have the ability to execute human clinical trials on the identified compound. After an IND filing, the remaining research services represent a separate unit of accounting which has standalone value upon delivery. With respect to the third Option Target where the Company retains U.S. rights, because the license terms restrict Celgene’s access to information through the completion of a Phase 1 trial that would otherwise be necessary for Celgene, or other market participants, to execute human clinical trials on the identified compound, the Company determined the licenses did not have standalone value apart from the research services Accordingly, for the third Option Target, the licenses and research services will be accounted for as a combined unit of accounting.

Because Celgene’s option rights with respect to the Option Targets are not considered substantive, any development milestone-based license fees associated with the Option Targets would be considered to be part of the total consideration for purposes of allocating the arrangement consideration. Accordingly, the Company has identified the total allocable arrangement consideration to be $106.6 million, as follows: (i) the remaining $21.6 million of deferred revenue under the original agreement, (ii) the $10.0 million upfront payment made in connection with the amended and restated agreement, and (iii) the $75.0 million of aggregate development milestone-based license fees associated with the Option Targets.

The allocable arrangement consideration has been allocated to the identified deliverables using the relative selling price method. The Company estimated the selling price of the license deliverable for each Option Target using management’s best estimate of selling price after considering potential future cash flows under each license and industry benchmarks on the probability of achieving development milestones. The Company then discounted these probability-weighted cash flows to their present value. The Company estimated the selling price of the research services to be provided in connection with the DOT1L Phase 1 clinical trial and related to each Option Target using management’s best estimate of selling price based on the Company’s estimated cost of providing the services plus an applicable profit margin of 10%, which is commensurate with observable market data for similar services.

Due to the stage of development of each of the option programs and considering the expected timing of delivery for those deliverables associated with each Option Target, the Company does not expect to recognize revenue related to the Option Targets for several years. The Company expects to recognize the allocated arrangement consideration as follows, subject to the limitation described in ASC 605-25-30-5:

•

Research and development services for DOT1L: The arrangement consideration allocated to the DOT1L program will be recognized as the research and development services are performed. The Company allocated $2.8 million to the remaining research services under the DOT1L program and will recognize this amount ratably through December 31, 2016, the estimated period that the research services will be provided.

•

Two Option Targets: The arrangement consideration allocated to these Option Targets of $70.2 million in the aggregate will be deferred until an IND filing. The Company does not expect to recognize any revenue related to the Option Targets during the next twelve months.

•

Third Option Target: The licenses and the research services provided through completion of a Phase 1 trial represent a single combined unit of accounting. Therefore, because a license is the last delivered item in this unit of accounting, the Company will defer all consideration allocated to this unit of accounting, $33.6 million, until the delivery of the license upon completion of a Phase 1 clinical trial. The Company does not expect to recognize any revenue related to the Option Targets during the next twelve months.

•

If any of the programs are terminated in accordance with the amended and restated agreement, any remaining deferred revenue will be reallocated among the undelivered items based upon their relative selling prices.

Collaboration Revenue

Through September 30, 2015, in addition to amounts allocated to Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock, the Company had recorded a total of $109.8 million in cash and accounts receivable under the Celgene agreement, as amended, including the $3.0 million implied premium on Celgene’s purchase of shares of the Company’s series C redeemable convertible preferred stock.

Through September 30, 2015, the Company has recognized $71.9 million of total collaboration revenue since the inception of the collaboration, including $0.4 million and $0.5 million in the three and nine months ended September 30, 2015, respectively, and $1.2 million and $4.1 million in the three and nine months ended September 30, 2014, respectively, and $6.8 million of total global development co-funding as a reduction to research and development expense since the inception of the collaboration, including $0.1 million and $1.0 million in the three and nine months ended September 30, 2015, respectively, and $1.2 million and $2.5 million in the three and nine months ended September 30, 2014, respectively, in the condensed consolidated statements of operations and comprehensive loss. As of September 30, 2015 and December 31, 2014, the Company had deferred revenue of $31.3 million and $21.7 million, respectively, related to this agreement.

GSK

In January 2011, the Company entered into a collaboration and license agreement with GSK, to discover, develop and commercialize novel small molecule HMT inhibitors directed to available targets from the Company’s platform. Under the terms of the agreement, the Company granted GSK exclusive worldwide license rights to HMT inhibitors directed to three targets. Additionally, as part of the research collaboration, the Company agreed to provide research and development services related to the licensed targets pursuant to agreed upon research plans during a research term that ended January 8, 2015. In March 2014, the Company and GSK amended certain terms of this agreement for the third licensed target, revising the license terms with respect to candidate compounds and amending the corresponding financial terms, including reallocating milestone payments and increasing royalty rates as to the third target. The Company substantially completed all research obligations under this agreement by the end of the first quarter of 2015 and completed the transfer of the remaining data and materials for these programs to GSK in the second quarter of 2015.

Agreement Structure

Under the agreement, the Company 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. The Company is eligible to receive up to $18.0 million in additional preclinical research and development milestone payments, up to $109.0 million in clinical development milestone payments, up to $275.0 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, 2015, the Company received a total of $53.0 million in cash under the GSK agreement, which the Company recognized as collaboration revenue in the condensed consolidated statements of operations and comprehensive loss, including $1.4 million in the nine months ended September 30, 2015 and $5.6 million and $22.0 million in the three and nine months ended September 30, 2014, respectively, including a $3.0 million preclinical research and development milestone achieved and recognized as collaboration revenue in the nine months ended September 30, 2014. As of December 31, 2014, the Company had deferred revenue of $1.4 million related to this agreement, which was fully recognized as collaboration revenue in the three months ended June 30, 2015.

Companion Diagnostics

Roche

In December 2012, Eisai and the Company entered into an agreement with Roche under which Eisai and the Company agreed to fund Roche’s development of a companion diagnostic to identify patients who possess certain point mutations in EZH2. At the same time, Eisai and the Company entered into a letter agreement pursuant to which Eisai agreed to be responsible for the development costs under the Roche agreement. In October 2013, this agreement was amended to include additional point mutations in EZH2. Under the terms of the amended agreement, Roche is to be paid up to a total of $21.5 million to develop and to make commercially available the companion diagnostic.

In connection with the March 2015 execution of the amended and restated collaboration and license agreement with Eisai, the Company and Eisai entered into an amended and restated letter agreement, pursuant to which the Company agreed to be responsible for up to $15.5 million of the remaining development costs under the agreement with Roche. Eisai continues to be responsible for up to $1.0 million of the remaining Japan-specific development costs under the agreement with Roche. The Company is recognizing the $15.5 million of remaining development costs under the agreement on a straight-line basis, through the end of the development term in 2020. In October 2015, the Company made a $3.0 million milestone payment to Roche under this agreement. Because the payment occurred subsequent to September 30, 2015, it is not reflected in the consolidated financial statements in this report.

Stock-Based Compensation

9 Months Ended

Sep. 30, 2015

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

8.	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 $1.6 million for the three months ended September 30, 2015 and 2014, respectively, and $7.8 million and $4.7 million for the nine months ended September 30, 2015 and 2014, respectively.

Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
	(In thousands)
Research and development	$	1,214	$	749	$	4,009	$	2,245
General and administrative		1,642		834		3,757		2,433

Total	$	2,856	$	1,583	$	7,766	$	4,678

Stock Options

The weighted-average fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $15.78 and $21.32 per option for those options granted during the three months ended September 30, 2015 and 2014, respectively, and $15.05 and $22.21 per option for those options granted during the nine months ended September 30, 2015 and 2014, respectively. Key assumptions used to apply this pricing model were as follows:


	Nine Months Ended September 30,
	2015			2014
Risk-free interest rate		1.6	%		1.6	%
Expected life of options		6.0 years			6.0 years
Expected volatility of underlying stock		83.7	%		92.4	%
Expected dividend yield		0.0	%		0.0	%

The following is a summary of stock option activity for the nine months ended September 30, 2015:


	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2014		2,959,506		$	10.66
Granted		1,145,555			21.23
Exercised		(547,189	)		1.60
Forfeited or expired		(439,091	)		18.81

Outstanding at September 30, 2015		3,118,781		$	15.09		6.4	$	12,862

Exercisable at September 30, 2015		1,333,544		$	8.03		3.4	$	11,291

As of September 30, 2015, there was $19.8 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, 2015:


	Number of Units		Weighted Average Grant Date Fair Value per Unit

Outstanding at December 31, 2014		—	$	—
Granted		37,313		18.49

Outstanding at September 30, 2015		37,313	$	18.49

As of September 30, 2015, there was $1.0 million of unrecognized compensation cost related to restricted stock units that are expected to vest, including $0.7 million of unrecognized compensation cost related to restricted stock units to be issued in the first quarter of 2016, pursuant to a February 2015 employment agreement with the Company’s chief financial officer, for which service is currently being provided. These costs are expected to be recognized over a weighted average remaining vesting period of 3.4 years.

Loss Per Share

9 Months Ended

Sep. 30, 2015

Earnings Per Share [Abstract]

Loss Per Share

9.	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,
	2015			2014			2015			2014
	(In thousands except per share data)
Net loss	$	(23,065	)	$	(19,700	)	$	(110,150	)	$	(39,970	)
Weighted average shares outstanding		41,461			33,676			39,204			32,607

Basic and diluted loss per share allocable to common stockholders	$	(0.56	)	$	(0.58	)	$	(2.81	)	$	(1.23	)

In March 2015, the Company issued 6,000,000 shares of common stock at a price of $20.75 per share in connection with a public offering. In April 2015, the Company issued and sold an additional 701,448 shares of common stock in connection with the March 2015 public offering pursuant to the underwriters’ option to purchase additional shares that the Company granted in connection with the public offering. The issuance of these shares contributed to a significant increase in the Company’s shares outstanding and weighted average shares outstanding for the three and nine months ended September 30, 2015 when compared to the comparable prior year periods.

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,
	2015		2014		2015		2014
	(In thousands)
Stock options		3,119		3,552		3,119		3,552
Unvested restricted stock units		37		—		37		—
Shares issuable under employee stock purchase plan		2		2		2		2

		3,158		3,554		3,158		3,554

Related Party Transactions

9 Months Ended

Sep. 30, 2015

Related Party Transactions

10.

Related Party Transactions

Celgene has made a series of equity investments in the Company, owning 3,674,640 shares of common stock representing 8.2% of the Company’s fully diluted equity and 8.8% of the voting interests of the Company as of September 30, 2015. Refer to Note 7, Collaborations for additional information regarding our original agreement with Celgene entered into in April 2012 and the amended and restated agreement with Celgene entered into in July 2015.

The Company recognized $0.4 million and $0.5 million of collaboration revenue in the three and nine months ended September 30, 2015, respectively, and $1.2 million and $4.1 million of collaboration revenue in the three and nine months ended September 30, 2014, respectively, related to the original and amended and restated agreements. As of September 30, 2015 and December 31, 2014, the Company’s deferred revenue balances included $31.3 million and $27.2 million related to the Celgene amended and restated agreement and original agreement, respectively. Additionally, in the three and nine months ended September 30, 2015, the Company recorded $0.1 million and $1.0 million, respectively, and in the three and nine months ended September 30, 2014 $1.2 million and $2.5 million, respectively, in global development co-funding from Celgene. As of September 30, 2015 and December 31, 2014, the Company recorded accounts receivable of $0.1 million and $1.1 million, respectively, from Celgene for global development co-funding.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References + Details

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2015

Accounting Policies [Abstract]

Property and Equipment

Property and Equipment

The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:


Asset Category		Useful Lives
Laboratory equipment		5 - 20 years
Office furniture and equipment		3 - 10 years or term of respective lease, if shorter
Leasehold improvements		3 - 10 years or term of respective lease, if shorter

Acquired In-Process Research and Development

Acquired In-Process Research and Development

There have been no other material changes to the significant accounting policies previously disclosed in the Company’s Annual Report.

Recent Accounting Pronouncements

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2015

Accounting Policies [Abstract]

Useful Lives for Property, Plant and Equipment

The Company calculates depreciation and amortization using the straight-line method over the following estimated useful lives:


Asset Category		Useful Lives
Laboratory equipment		5 - 20 years
Office furniture and equipment		3 - 10 years or term of respective lease, if shorter
Leasehold improvements		3 - 10 years or term of respective lease, if shorter

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2015

Fair Value Disclosures [Abstract]

Summary of Company's Financial Assets Recognized at Fair Value

As of September 30, 2015 and December 31, 2014, the Company’s financial assets recognized at fair value consisted of the following:


	Fair Value as of September 30, 2015
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	196,975	$	196,975	$	—	$	—

Total	$	196,975	$	196,975	$	—	$	—


	Fair Value as of December 31, 2014
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	184,257	$	184,257	$	—	$	—

Total	$	184,257	$	184,257	$	—	$	—

Accrued Expenses (Tables)

9 Months Ended

Sep. 30, 2015

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consisted of the following:


	September 30, 2015		December 31, 2014
	(In thousands)
Employee compensation and benefits	$	2,780	$	2,623
Research and development and professional expenses		9,301		4,420

Accrued expenses	$	12,081	$	7,043

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2015

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

Future minimum equipment lease payments under this capital lease, net of imputed interest, as of September 30, 2015 are as follows:


	(In thousands)
Future minimum lease payments in year ending December 31:
2015	$	166
2016		665
2017		665
2018		112

Total future minimum lease payments		1,608
Less: Amount representing imputed interest on equipment lease		(185	)

Capital lease obligation	$	1,423

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock-Based Compensation Expense

Stock-based compensation expense is classified in the condensed consolidated statements of operations and comprehensive loss as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
	(In thousands)
Research and development	$	1,214	$	749	$	4,009	$	2,245
General and administrative		1,642		834		3,757		2,433

Total	$	2,856	$	1,583	$	7,766	$	4,678

Assumptions Used to Applying Pricing Model

Key assumptions used to apply this pricing model were as follows:


	Nine Months Ended September 30,
	2015			2014
Risk-free interest rate		1.6	%		1.6	%
Expected life of options		6.0 years			6.0 years
Expected volatility of underlying stock		83.7	%		92.4	%
Expected dividend yield		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, 2015:


	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2014		2,959,506		$	10.66
Granted		1,145,555			21.23
Exercised		(547,189	)		1.60
Forfeited or expired		(439,091	)		18.81

Outstanding at September 30, 2015		3,118,781		$	15.09		6.4	$	12,862

Exercisable at September 30, 2015		1,333,544		$	8.03		3.4	$	11,291

Summary of Restricted Stock Activity

The following is a summary of restricted stock unit activity for the nine months ended September 30, 2015:


	Number of Units		Weighted Average Grant Date Fair Value per Unit

Outstanding at December 31, 2014		—	$	—
Granted		37,313		18.49

Outstanding at September 30, 2015		37,313	$	18.49

Loss Per Share (Tables)

9 Months Ended

Sep. 30, 2015

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,
	2015			2014			2015			2014
	(In thousands except per share data)
Net loss	$	(23,065	)	$	(19,700	)	$	(110,150	)	$	(39,970	)
Weighted average shares outstanding		41,461			33,676			39,204			32,607

Basic and diluted loss per share allocable to common stockholders	$	(0.56	)	$	(0.58	)	$	(2.81	)	$	(1.23	)

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,
	2015		2014		2015		2014
	(In thousands)
Stock options		3,119		3,552		3,119		3,552
Unvested restricted stock units		37		—		37		—
Shares issuable under employee stock purchase plan		2		2		2		2

		3,158		3,554		3,158		3,554

Overview and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended
	Apr. 30, 2015	Mar. 31, 2015	Sep. 30, 2015	Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Common stock issued during period	701,448	6,000,000
Common stock price per share	$ 20.75	$ 20.75
Net proceeds from public offering	$ 13,700	$ 117,000	$ 130,712	$ 101,283

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets Acquired (Detail)	9 Months Ended
	Sep. 30, 2015
Laboratory Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	5 years
Laboratory Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	20 years
Office Furniture and Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Office Furniture and Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years
Leasehold Improvements [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	3 years
Leasehold Improvements [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	10 years

Fair Value Measurements - Summary of Company's Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 196,975	$ 184,257
Total	196,975	184,257
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	196,975	184,257
Total	196,975	184,257
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total	0	0
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total	$ 0	$ 0

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accrued Liabilities, Current [Abstract]
Employee compensation and benefits	$ 2,780	$ 2,623
Research and development and professional expenses	9,301	4,420
Accrued expenses	$ 12,081	$ 7,043

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Line Items]
Federal income tax provision or benefit	$ 0		$ 0
State income tax provision or benefit	$ 0		$ 0
Maximum [Member]
Income Tax Disclosure [Line Items]
Income tax expense		$ 100,000		$ 100,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Contractual Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	$ 166
2016	665
2017	665
2018	112
Total future minimum lease payments	1,608
Less: Amount representing imputed interest on equipment lease	(185)
Capital lease obligation	$ 1,423

Commitments and Contingencies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) ft²
Operating Lease [Member] \| North Carolina [Member]
Other Commitments [Line Items]
Lease expiration date	Jul. 31, 2017
Total future minimum lease payments	$ 0.2
Rental space \| ft²	4,000
Roche [Member]
Other Commitments [Line Items]
Remaining development costs	$ 15.5
Remaining development costs	1.0
Eisai [Member]
Other Commitments [Line Items]
Clinical development milestone payments	20.0
Regulatory milestone payments	$ 50.0

Collaborations - Additional Information (Detail)		1 Months Ended			3 Months Ended		9 Months Ended		36 Months Ended	42 Months Ended	57 Months Ended
Collaborations - Additional Information (Detail)	Jul. 08, 2015 USD ($) OptionTargets	Oct. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue					$ 358,000	$ 8,177,000	$ 2,005,000	$ 31,062,000
Upfront payment made							40,000,000
Eisai [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue						1,400,000		5,000,000
Deferred revenue												$ 0
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]
Collaboration revenue					400,000	1,200,000	500,000	4,100,000		71,900,000
Deferred revenue					31,300,000		31,300,000			31,300,000	31,300,000	21,700,000
Company received upfront payment										65,000,000
Proceeds from redeemable convertible preferred stock										25,000,000
Upfront payment recorded										68,000,000
Clinical development milestone achieved										25,000,000
Global development co-funding					100,000	1,200,000	1,000,000	2,500,000		6,800,000
Number of option targets \| OptionTargets	3
Deferred revenue recognized	$ 21,600,000
Upfront payment made as part of the amendment	10,000,000
License and research services related cost	106,600,000
Cash and accounts receivable										109,800,000
Convertible preferred stock issued, premium value										3,000,000
Celgene [Member] \| Third Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost	33,600,000
Celgene [Member] \| License and Pre-IND Services [Member] \| Two Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
License and research services related cost	$ 70,200,000
Number of option targets with no standalone value \| OptionTargets	2
Celgene [Member] \| Minimum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Option targets under amended and restated agreement extended period	3 years
Celgene [Member] \| United States [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets \| OptionTargets	2
Celgene [Member] \| Non-US [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets \| OptionTargets	1
Celgene [Member] \| DOT1L [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Clinical development milestone payment					35,000,000		35,000,000			35,000,000	35,000,000
Additional milestone payments					100,000,000		100,000,000			100,000,000	100,000,000
License and research services related cost	$ 2,800,000
Applicable profit margin	10.00%
Celgene [Member] \| DOT1L [Member] \| License and Pre-IND Services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Number of option targets with standalone value \| OptionTargets	2
Celgene [Member] \| Available Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments					100,000,000		100,000,000			100,000,000	100,000,000
Additional payments					65,000,000		65,000,000			65,000,000	65,000,000
Celgene [Member] \| Option Targets [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Company received upfront payment							10,000,000
Development milestone and license payments associated with the Option Targets	$ 75,000,000
Celgene [Member] \| Option Targets [Member] \| Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Additional milestone payments					365,000,000		365,000,000			365,000,000	365,000,000
Development milestone and license payments associated with the Option Targets							75,000,000
Sales-based milestone payments					170,000,000		170,000,000			170,000,000	170,000,000
GSK [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Collaboration revenue						$ 5,600,000	1,400,000	22,000,000
Deferred revenue												$ 1,400,000
Upfront payment recorded				$ 3,000,000					$ 20,000,000
Clinical development milestone payment					109,000,000		109,000,000			109,000,000	109,000,000
Additional milestone payments					275,000,000		275,000,000			275,000,000	275,000,000
Sales-based milestone payments					218,000,000		218,000,000			218,000,000	218,000,000
Cash and accounts receivable											53,000,000
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
Preclinical research and development milestone achieved								$ 3,000,000
Roche [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remaining unpaid milestone payments					15,500,000		15,500,000			15,500,000	15,500,000
Total development milestone payments					21,500,000		21,500,000			21,500,000	21,500,000
Remaining unpaid milestone payments					$ 1,000,000		$ 1,000,000			$ 1,000,000	$ 1,000,000
Development agreement term							2020
Roche [Member] \| Subsequent Event [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Milestone amount paid		$ 3,000,000

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options and restricted stock	$ 2,856	$ 1,583	$ 7,766	$ 4,678
Weighted-average fair value of options granted	$ 15.78	$ 21.32	$ 15.05	$ 22.21
Unrecognized compensation cost related to stock options	$ 19,800		$ 19,800
Expected weighted average period for recognition of compensation cost			2 years 9 months 18 days
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			3 years 4 months 24 days
Unrecognized compensation cost related to restricted stock units	1,000		$ 1,000
Unrecognized compensation cost related to restricted stock units to be issued	$ 700		$ 700

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 2,856	$ 1,583	$ 7,766	$ 4,678
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	1,214	749	4,009	2,245
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 1,642	$ 834	$ 3,757	$ 2,433

Stock-Based Compensation - Assumptions Used to Applying Pricing Model (Detail) - Employee Stock Option [Member]	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.60%	1.60%
Expected life of options	6 years	6 years
Expected volatility of underlying stock	83.70%	92.40%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding, Beginning balance \| shares	2,959,506
Number of Options, Granted \| shares	1,145,555
Number of Options, Exercised \| shares	(547,189)
Number of Options, Forfeited or expired \| shares	(439,091)
Number of Options, Outstanding, Ending balance \| shares	3,118,781
Number of Options, Exercisable \| shares	1,333,544
Weighted Average Exercise Price per Share, Outstanding, Beginning balance \| $ / shares	$ 10.66
Weighted Average Exercise Price per Share, Granted \| $ / shares	21.23
Weighted Average Exercise Price per Share, Exercised \| $ / shares	1.60
Weighted Average Exercise Price per Share, Forfeited or expired \| $ / shares	18.81
Weighted Average Exercise Price per Share, Outstanding, Ending balance \| $ / shares	15.09
Weighted Average Exercise Price per Share, Exercisable \| $ / shares	$ 8.03
Weighted Average Remaining Contractual Term (In Years), Outstanding	6 years 4 months 24 days
Weighted Average Remaining Contractual Term (In Years), Exercisable	3 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding \| $	$ 12,862
Aggregate Intrinsic Value, Exercisable \| $	$ 11,291

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) - Unvested Restricted Stock Units [Member]	9 Months Ended
	Sep. 30, 2015 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Granted \| shares	37,313
Number of Units, Outstanding, Ending balance \| shares	37,313
Weighted Average Grant Date Fair Value per Units, Granted \| $ / shares	$ 18.49
Weighted Average Grant Date Fair Value per Units, Ending balance \| $ / shares	$ 18.49

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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net loss	$ (23,065)	$ (19,700)	$ (110,150)	$ (39,970)
Weighted average shares outstanding	41,461	33,676	39,204	32,607
Basic and diluted loss per share allocable to common stockholders	$ (0.56)	$ (0.58)	$ (2.81)	$ (1.23)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Loss Per Share - Additional Information (Detail) - $ / shares	1 Months Ended
	Apr. 30, 2015	Mar. 31, 2015
Earnings Per Share [Abstract]
Common stock issued during period	701,448	6,000,000
Common stock price per share	$ 20.75	$ 20.75

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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted loss per share	3,158	3,554	3,158	3,554
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	3,119	3,552	3,119	3,552
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	37		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	2	2	2	2

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Equity investments in common stock	41,698,000		41,698,000		34,426,000
Beneficial Owner [Member]
Related Party Transaction [Line Items]
Equity investments in common stock	3,674,640		3,674,640
Diluted equity	8.20%		8.20%
Percentage of voting interests	8.80%		8.80%
Collaboration revenue related to agreement	$ 0.4	$ 1.2	$ 0.5	$ 4.1
Deferred revenue	31.3		31.3		$ 27.2
Global development co-funding from Celgene	0.1	$ 1.2	1.0	$ 2.5
Accounts receivable related to collaboration arrangement	$ 0.1		$ 0.1		$ 1.1