bluebird bio (BLUE) 10-Q 12 Aug 14 2014 Q2 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	6 Months Ended
	Jun. 30, 2014	Jul. 29, 2014
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Jun-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q2	'
Entity Registrant Name	'bluebird bio, Inc.	'
Entity Central Index Key	'0001293971	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Non-accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	28,595,796

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$175,710	$206,279
Deferred tax assets	3,106	693
Prepaid expenses and other current assets	4,053	5,015
Total current assets	182,869	211,987
Property and equipment, net	12,857	10,920
Intangible assets	30,100	'
Goodwill	13,128	'
Restricted cash and other non-current assets	1,668	1,483
Total assets	240,622	224,390
Current liabilities:	'	'
Accounts payable	2,586	4,359
Accrued expenses and other current liabilities	11,670	5,175
Deferred revenue, current portion	25,291	25,340
Total current liabilities	39,547	34,874
Deferred rent, net of current portion	8,056	6,740
Deferred revenue, net of current portion	17,708	30,208
Contingent consideration, net of current portion	6,097	'
Deferred tax liabilities	3,106	693
Other non-current liabilities	352	208
Total liabilities	74,866	72,723
Commitments and contingencies (Note 4)	'	'
Stockholders' equity:	'	'
Preferred stock, $0.01 par value, 5,000 shares authorized; 0 shares issued and outstanding at June 30, 2014 and December 31, 2013	'	'
Common stock, $0.01 par value, 125,000 shares authorized; 25,077 and 23,940 shares issued and outstanding at June 30, 2014 and December 31, 2013, respectively	251	239
Additional paid-in capital	276,315	250,103
Accumulated deficit	-110,810	-98,675
Total stockholders' equity	165,756	151,667
Total liabilities and stockholders' equity	$240,622	$224,390

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, except Per Share data, unless otherwise specified
Statement Of Financial Position [Abstract]	'	'
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	5,000	5,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	125,000	125,000
Common stock, shares issued	25,077	23,940
Common stock, shares outstanding	25,077	23,940

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenue:	'	'	'	'
Collaboration revenue	$6,250	$6,249	$12,500	$7,291
Research and license fees	85	85	170	170
Total revenue	6,335	6,334	12,670	7,461
Operating expenses:	'	'	'	'
Research and development	13,931	7,247	25,394	12,531
General and administrative	5,738	3,281	11,277	5,605
Total operating expenses	19,669	10,528	36,671	18,136
Loss from operations	-13,334	-4,194	-24,001	-10,675
Other income (expense), net	11	-389	69	-452
Loss before income taxes	-13,323	-4,583	-23,932	-11,127
Benefit from income taxes	11,797	'	11,797	'
Net loss	-1,526	-4,583	-12,135	-11,127
Net loss per share - basic and diluted:	($0.06)	($2.13)	($0.50)	($8.94)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	24,474	2,151	24,312	1,244
Comprehensive loss	($1,526)	($4,583)	($12,135)	($11,127)

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013
Operating activities	'	'
Net loss	($12,135)	($11,127)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:	'	'
Noncash benefit on release of tax valuation allowance	-11,797	'
Depreciation and amortization	1,017	304
Stock-based compensation expense	4,843	2,318
Issuance of restricted common stock in exchange for consulting services to non-employees	168	'
Re-measurement of warrants	'	440
Loss on disposal of equipment	'	2
Changes in operating assets and liabilities:	'	'
Prepaid expenses and other assets	854	-906
Accounts payable	-2,702	-1,416
Accrued expenses and other liabilities	7,674	617
Deferred revenue	-12,670	67,539
Deferred rent	1,490	-6
Net cash (used in) provided by operating activities	-23,258	57,765
Investing activities	'	'
Restricted cash	'	-1,253
Purchase of property and equipment	-4,534	-1,265
Acquisition of business, net of cash acquired	-4,673	'
Net cash used in investing activities	-9,207	-2,518
Financing activities	'	'
Proceeds from IPO, net of issuance costs	'	106,180
Repayment of nonrecourse note collateralized by restricted stock	'	344
Proceeds from exercise of stock options	1,896	65
Net cash provided by financing activities	1,896	106,589
(Decrease) increase in cash and cash equivalents	-30,569	161,836
Cash and cash equivalents at beginning of period	206,279	67,011
Cash and cash equivalents at end of period	175,710	228,847
Non-cash investing and financing activities:	'	'
Purchases of property and equipment included in accounts payable and accrued expenses	344	281
Offering costs included in accounts payable and accrued expenses	45	1,259
Reclassification of warrants to additional paid-in capital	'	655
Conversion of preferred stock to common stock upon closing of IPO	'	119,841
Receivable for stock option exercise proceeds	$75	'

Description_of_the_business

Description of the business	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Description of the business	'
	1. Description of the business
	bluebird bio, Inc. (the “Company” or “bluebird”) was incorporated in Delaware on April 16, 1992, and is headquartered in Cambridge, Massachusetts. The Company was formed to develop, manufacture and market therapies to safely and effectively deliver genes useful in the treatment of serious human diseases. Since its inception, the Company has devoted substantially all of its resources to its development efforts relating to its product candidates, including activities to manufacture product in compliance with good manufacturing practices (“GMP”), conduct clinical studies of its product candidates, provide general and administrative support for these operations and protect its intellectual property.
	On June 30, 2014, the Company acquired all of the outstanding capital stock of Precision Genome Engineering, Inc. (“Pregenen”) and in connection therewith, obtained the rights to Pregenen’s gene editing and cell signaling technology. See Note 6, “Business combinations,” for additional information.

Summary_of_significant_account

Summary of significant accounting policies and basis of presentation	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Summary of significant accounting policies and basis of presentation	'
	2. Summary of significant accounting policies and basis of presentation
	Basis of presentation and principles of consolidation
	The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended June 30, 2014 and 2013.
	The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on March 5, 2014.
	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Precision Genome Engineering, Inc., bluebird bio France – SARL, bluebird bio Australia Pty Ltd. and bluebird bio Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. The assets acquired and liabilities assumed in connection with the Company’s acquisition of Pregenen were recorded at their fair values as of June 30, 2014, the date of the acquisition, and the operating results of Pregenen have been consolidated with those of the Company from the date of acquisition. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. All material long-lived assets of the Company reside in the United States.
	Summary of accounting policies
	The significant accounting policies described in the Company’s audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, which are included in the Annual Report on Form 10-K, have had no material changes during the six months ended June 30, 2014, except as noted below:
	Business combinations
	On June 30, 2014, the Company completed its acquisition of Pregenen for total consideration of $30,991, consisting of cash consideration of $5,093, common stock consideration of $19,348 and contingent consideration with an estimated fair value of $6,550. The estimated fair value of the contingent consideration is based upon significant assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and discount rates. The estimated fair value could materially differ from actual values or fair values determined using different assumptions. See Note 3, “Fair value measurements,” for additional information.
	This transaction was accounted for as a business combination under the acquisition method of accounting. Accordingly, the tangible assets and identifiable intangible assets acquired and liabilities assumed were recorded at fair value as of the date of acquisition, with the remaining purchase price recorded as goodwill. The estimated fair values of the assets acquired and liabilities assumed at the date of acquisition are summarized in Note 6, “Business combinations.” The estimated fair values of acquired assets and assumed liabilities were determined using the methods discussed in the following paragraphs and require significant judgment and estimates, which could materially differ from actual values and fair values determined using different methods or assumptions.
	The purchase price allocation was initially prepared on a preliminary basis and is subject to change as additional information becomes available concerning the fair value and tax basis of the assets acquired and liabilities assumed and contingent consideration. Any adjustments to the purchase price allocations are made as soon as practicable but no later than one year from the acquisition date.
	Goodwill
	Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company has not recognized any impairment charges related to goodwill.
	Intangible assets
	Intangible assets consist of acquired core technology with finite lives. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives. The Company evaluates the potential impairment of intangible assets if events or changes in circumstances indicate that the carrying amount of the assets may not be fully recoverable or that the useful lives of these assets are no longer appropriate. The impairment test is based on a comparison of the undiscounted cash flows expected to be generated from the use of the asset group and its eventual disposition to the carrying value of the asset group. If impairment is indicated, the asset is written down by the amount by which the carrying value of the asset exceeds the related fair value of the asset. The Company has not recognized an impairment charge related to intangible assets.
	Contingent consideration
	Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and record within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s clinical programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 3, “Fair value measurements,” for additional information.
	Use of estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates are used in the following areas, among others: acquisition-date fair value and subsequent fair value estimates used to assess impairment of long-lived assets, including goodwill and intangible assets, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes.
	Fair value of financial instruments
	The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC 820, Fair Value Measurements and Disclosures, establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:
	Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.
	Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.
	Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.
	The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities.
	Other Income and Expense
	In March 2014, the Company received an award of $306 of tax incentives from the Massachusetts Life Sciences Center, which will allow the Company to monetize approximately $276 of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental fifteen employees and to maintain the additional headcount through at least December 31, 2018. Failure to do so could result in the Company being required to repay some or all of these incentives. As the Company met the additional headcount during the quarter ended June 30, 2014, the Company has deferred and is amortizing the benefit of this monetization on a straight-line basis over the five-year performance period.
	Net Income (Loss) Per Share
	Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, warrants, and acquisition holdback shares using the treasury stock method.
	Recent accounting pronouncements
	During the quarter ended June 30, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU No. 2014-09”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2017 and early adoption is not permitted for public entities. The Company is currently evaluating the potential impact that ASU No. 2014-09 may have on its financial position and results of operations.

Fair_value_measurements

Fair value measurements	6 Months Ended
	Jun. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair value measurements	'
	3. Fair value measurements
	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2014 and December 31, 2013:
	Description	Total		Quoted		Significant		Significant
				prices in		other		unobservable
				active		observable		inputs
				markets		inputs		(Level 3)
				(Level 1)		(Level 2)
	30-Jun-14
	Assets:
	Cash and cash equivalents	$	175,710	$	175,710	$	—	$	—
	Total assets	$	175,710	$	175,710	$	—	$	—
	Liabilities:
	Contingent consideration	$	6,550	$	—	$	—	$	6,550
	Total liabilities	$	6,550	$	—	$	—	$	6,550
	31-Dec-13
	Total cash and cash equivalents	$	206,279	$	206,279	$	—	$	—
	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. As of June 30, 2014 and December 31, 2013, cash and cash equivalents comprise funds in cash and money market accounts.
	Contingent consideration
	In connection with the acquisition of Pregenen, the Company recorded contingent consideration pertaining to the amounts potentially payable to Pregenen’s former equityholders pursuant to the Stock Purchase Agreement (the “Stock Purchase Agreement”) by and among the Company, Pregenen and Pregenen’s former equityholders. Contingent consideration is measured at fair value and is based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the condensed consolidated statements of operations and comprehensive loss.
	Contingent consideration may change significantly as development progresses and additional data are obtained, impacting the Company’s assumptions regarding probabilities of successful achievement of related milestones used to estimate the fair value of the liability and the timing in which they are expected to be achieved. In evaluating the fair value information, considerable judgment is required to interpret the market data used to develop the estimates. The estimates of fair value may not be indicative of the amounts that could be realized in a current market exchange. Accordingly, the use of different market assumptions and/or different valuation techniques could result in materially different fair value estimates.
	The significant unobservable inputs used in the measurement of fair value of the Company’s contingent consideration are probabilities of successful achievement of preclinical, clinical and commercial milestones, the period in which these milestones are expected to be achieved ranging from 2015 to 2026 and discount rates ranging from 10% to 15%. Significant increases or decreases in any of the probabilities of success would result in a significantly higher or lower fair value measurement, respectively. Significant increases or decreases in these other inputs would result in a significantly lower or higher fair value measurement, respectively.
	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations which include Level 3 inputs:
		Six months ended
		June 30, 2014
	Beginning balance	$	—
	Additions		6,550
	Changes in fair value		—
	Payments		—
	Ending balance	$	6,550
	As of June 30, 2014, $453 of the fair value of the Company’s total contingent consideration obligations was reflected as components of accrued expenses and other current liabilities within the condensed consolidated balance sheets with the remaining balances reflected as a non-current liability.

Commitments_and_contingencies

Commitments and contingencies	6 Months Ended
	Jun. 30, 2014
Commitments And Contingencies Disclosure [Abstract]	'
Commitments and contingencies	'
	4. Commitments and contingencies
	On June 3, 2013, the Company entered into a nine-year building lease for approximately 43,600 square feet of space for its new corporate headquarters at 150 Second Street, Cambridge, Massachusetts, which commenced in December 2013. The lease has monthly lease payments of $209 the first 12 months with annual rent escalations thereafter and provided a rent abatement of $209 per month for the first six months. The Company has the option to extend this lease by an additional five years. As the Company obtained access to the newly leased space on July 22, 2013 in order to begin the build-out, this is considered the lease commencement date for accounting purposes, thus rent expense began on this date and is recognized on a straight-line basis over the term of the lease. In addition, the lease provided a contribution from the landlord towards the initial build-out of the space of up to $6,538. The Company capitalizes the leasehold improvements as property and equipment and records the landlord incentive payments received as deferred rent and amortizes these amounts as reductions to rent expense over the lease term. In accordance with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $1,253, naming the landlord as beneficiary. This letter of credit may be reduced to $1,044, $835, and $627 upon the rent commencement date and the first and second anniversaries of the rent commencement date, respectively. The Company relocated into its new corporate headquarters in December 2013 and ceased use of its former facility during the first quarter of 2014. During the first quarter of 2014, the Company fully subleased its former facility, which decreased the rent abatement available under its new lease. The lease for the Company’s former headquarters, located at 840 Memorial Drive, Cambridge, Massachusetts, expires on March 31, 2015.
	On June 9, 2014, the Company amended its lease agreement to add an additional 9,869 square feet of space for its corporate headquarters at 150 Second Street, Cambridge, Massachusetts. The expansion increases the monthly lease payments by $47 beginning in September 2014 with rent escalations thereafter. In addition, the lease amendment provides a contribution from the landlord towards the build-out of the additional space of up to $1,234.
	The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at June 30, 2014 and December 31, 2013 or royalties on future sales of specified products.
	The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with claims by any third party with respect to the Company’s products or business activities. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.
	The Company’s wholly-owned subsidiary bluebird bio France – SARL participates in the French Crédit d’Impôt Recherche (“CIR”) program which allows companies to monetize up to 30% of eligible research expenses. The Company received aggregate reimbursement of $807 related to years 2010 through 2012. The Company recognized these amounts as reductions to research and development expense in the periods incurred. The years 2010 through 2013 are open and subject to examination. The Company has applied for $873 related to 2013, which is classified as current assets and has not yet applied for $566 related to the six months ended June 30, 2014 which are classified as non-current assets within the condensed consolidated balance sheets as of June 30, 2014. The Company received a notice from the Ministère de l’Enseignement Supérieur et de la Recherche in March 2014 inquiring of the Company regarding the eligibility of such research expenses pursuant to the CIR program. Based on the nature of the research activities conducted by bluebird bio France, SARL, the Company has concluded that it is probable that these receivables are collectable. However, any adjustment as a result of the inquiry would result in a refund of the amounts previously received or impairment of the Company’s receivable.
	In June 2014, the Company acquired Pregenen. The Company may be required to make up to an additional $135,000 in future contingent cash payments upon the achievement of certain preclinical, clinical and commercial milestones related to the Pregenen technology, of which $15,000 relates to preclinical milestones, $20,100 relates to clinical milestones and $99,900 relates to commercial milestones. In accordance with accounting for business combinations guidance, contingent consideration liabilities are required to be recognized on the condensed consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain preclinical, clinical and commercial milestones, the expected timing in which these milestones will be achieved and discount rates. The use of different assumptions could result in materially different estimates of fair value. See Note 3, “Fair value measurements,” and Note 6, “Business combinations,” for additional information.

Significant_agreements

Significant agreements	6 Months Ended
	Jun. 30, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]	'
Significant agreements	'
	5. Significant agreements
	Celgene Corporation
	Summary of the Collaboration Agreement
	On March 19, 2013, the Company entered into a Master Collaboration Agreement (the “Collaboration Agreement”) with Celgene to discover, develop and commercialize potentially disease-altering gene therapies in oncology. The collaboration is focused on applying gene therapy technology to genetically modify a patient’s own T cells, known as chimeric antigen receptor, or CAR, T cells, to target and destroy cancer cells. Additionally, on March 19, 2013, the Company entered into a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with Celgene pursuant to which the Company obtained a sublicense to certain intellectual property from Celgene, originating under Celgene’s license from Baylor College of Medicine, for use in the collaboration.
	Under the terms of the Collaboration Agreement, the Company received a $75,000 up-front, non-refundable cash payment. The Company will be responsible for conducting discovery, research and development activities through completion of Phase I clinical studies, if any, during the initial term of the agreement, or three years. The collaboration is governed by a joint steering committee (“JSC”) formed by an equal number of representatives from the Company and Celgene. The JSC will, among other activities, review the collaboration program, review and evaluate product candidates and approve regulatory plans. In addition to the JSC, the Collaboration Agreement provides that the Company and Celgene each appoint representatives to a patent committee, which is responsible for managing the intellectual property developed and used during the collaboration.
	Prior to expiration of the initial term of the Collaboration Agreement, Celgene has two options to extend the term, through March 19, 2019, with the payment of significant extension fees. Separately, Celgene has an option to license an unlimited number of product candidates resulting from the collaboration during a period commencing upon execution of the Collaboration Agreement and continuing through a specified period following the completion of Phase I clinical studies for each individual product candidate. In the event such option is exercised, the Company would grant Celgene an exclusive worldwide license to develop and commercialize such product candidate. Upon exercise of the option to license a product candidate, Celgene is required to pay an option fee, which is subject to reduction if the Company elects to co-develop and co-promote such product candidate in the United States. For any product candidates licensed by Celgene, the Company may be responsible, at Celgene’s election, to continue performing certain development activities contemplated as part of the collaboration plan. If Celgene does not exercise its option with respect to a product candidate prior to the expiration of the applicable option period (each a “declined product candidate”), then the Company has the right to develop the product candidate outside the scope of the collaboration, subject to a Celgene opt-in right to obtain a license to that declined product candidate for significant additional cash consideration. The opt-in right exists through a specified period following the completion of a pivotal study for the specific declined product candidate and functions in the same manner as the option to license any other product candidates resulting from the collaboration.
	In addition, Celgene would be required to make certain milestone payments upon the achievement of specified clinical, regulatory and commercial events. For each product candidate that is licensed by Celgene, the Company would be eligible to receive per product up to $20,000 in option fees, up to $10,000 in clinical milestone payments, up to $117,000 in regulatory milestone payments and up to $78,000 in commercial milestone payments. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon approval to market a product candidate by the FDA or other global regulatory authorities. Commercial milestone payments are triggered upon the first commercial sale of an approved pharmaceutical product and when an approved pharmaceutical product reaches certain defined levels of net sales by the licensee or receives approval to be marketed by certain global regulatory authorities in a specified number of countries outside of the United States. In addition, to the extent any of the product candidates licensed by Celgene are commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to mid-teens based on a percentage of net sales. Royalty payments are subject to certain reductions, including for any royalty payments required to be made by Celgene to acquire patent rights, with an aggregate minimum floor. The Company is not eligible to receive either milestone payments or royalty payments unless and until Celgene exercises its option to license a product candidate resulting from the collaboration whereupon the parties will execute a license agreement, the terms of which are included as part of the collaboration arrangement.
	Additionally, the Company may elect to co-develop and co-promote product candidates licensed by Celgene. If the Company elects to co-develop and co-promote a product candidate, then the parties would share equally in all costs incurred relating to the development, commercialization and manufacture of the product candidate within the United States and share equally in the profits generated by such product candidate in the United States. Additionally, if the Company elects to co-develop and co-promote a product candidate, then the option fees, milestones and royalties would decrease compared to those described above. Under this scenario, the Company would receive per product up to $10,000 in option fees, up to $10,000 in clinical milestone payments and outside of the United States, up to $54,000 in regulatory milestone payments and up to $36,000 in commercial milestone payments. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon approval to market a product candidate by global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by the licensee or receives approval to be marketed by certain global regulatory authorities in a specified number of countries outside the United States. In addition, to the extent any of the product candidates licensed by Celgene and co-developed and co-promoted by the Company are commercialized, the Company would be entitled to receive tiered royalty payments ranging from the mid-single digits to mid-teens based on a percentage of net sales from sales generated outside of the United States. Royalty payments are subject to certain reductions, including for any royalty payments required to be made by Celgene to acquire patent rights, with an aggregate minimum floor. The Company is not eligible to receive profit share payments, milestone payments or royalty payments unless and until Celgene exercises its option to license a product candidate resulting from the collaboration whereupon the parties will execute a co-development, co-promote and profit share agreement, the terms of which are included as part of the collaboration arrangement.
	In the event Celgene elects to license a product candidate discovered and developed as part of the Collaboration Agreement, Celgene would be solely responsible for all costs and expenses of manufacturing and supplying any product candidates. Subject to customary back-up supply rights granted to Celgene, the Company has the sole right to manufacture or have manufactured supplies of vectors and associated payloads manufactured for incorporation into the associated product candidate. Celgene would reimburse the Company for the costs incurred to manufacture and supply such vectors and associated payloads, plus a modest mark-up. The Company is not obligated to manufacture or have manufactured supplies of vectors and associated payloads for incorporation into an optioned product candidate unless and until Celgene exercises its option to license a product candidate resulting from the collaboration whereupon the parties will execute a separate manufacturing and supply agreement.
	The Collaboration Agreement may be terminated by either the Company or Celgene, upon written notice, in the event of the other party’s uncured material breach. Celgene may terminate the Collaboration Agreement for any reason upon written notice to the Company. If the Collaboration Agreement is terminated, rights to product candidates in development at the time of such termination will be allocated to the parties through a mechanism included in the Collaboration Agreement. In addition, if Celgene terminates the Collaboration Agreement as a result of a breach by the Company, then any then-existing co-development and co-promotion agreement will be automatically terminated and replaced with a license agreement for such product candidate and any amounts payable by Celgene under any then-existing product license agreements will be reduced.
	Call Option
	During the initial three-year term of the collaboration and, if extended, during the first two-year extension term of the collaboration, in the event that the Company engages in a change in control transaction, including for such purposes a merger or consolidation of the Company or the sale of all or substantially all of the Company’s assets, or if another person or entity or group of persons or entities acquires at least 50% of the Company’s voting capital stock, then Celgene has the right, but not the obligation, to terminate the Collaboration Agreement and obtain perpetual, non-terminable, worldwide, exclusive, fully paid-up licenses to all, but not less than all, of the product candidates previously identified under the Collaboration Agreement (the “Call Option”). Under the Call Option, the product candidates to which Celgene would have the right to acquire licenses include any product candidate previously licensed out of the collaboration during the term of the collaboration, any product candidate for which the Company has exercised the right to co-develop and co-promote within the United States, any product candidate for which Celgene previously declined its option to obtain a license and any product candidate for which at least in vivo efficacy studies have been initiated or authorized by the JSC. The purchase price for such licenses would be based on the fair value of these rights received and obligations assumed determined pursuant to a binding arbitration process.
	In addition, during the initial three-year term of the collaboration, but not during any extension term, in the event that Celgene exercises the Call Option, in addition to the right to acquire the fully paid-up licenses described above, Celgene would obtain a perpetual, non-terminable, worldwide, exclusive license to the Company’s intellectual property to develop one or more CAR T cell products targeting one or more oncology associated target antigens for the remainder of the initial collaboration term. Following the initial collaboration term, the license to the Company’s intellectual property is limited to target antigens identified by Celgene promptly following the initial collaboration term for which Celgene reasonably intends to develop CAR T cell products. There is no limit to the number of oncology-related target antigens Celgene may select under this license. Upon commercialization of any such product candidate so licensed by Celgene, Celgene would be obligated to pay the Company a specified milestone payment upon regulatory approval and a percentage of net sales as a royalty.
	The Company concluded that the value of the Call Option is immaterial based primarily on the probability that the Call Option would become exercisable.
	Accounting Analysis
	The Company’s arrangement with Celgene contains the following deliverables: (i) discovery, research and development services, (ii) participation on the JSC and (iii) participation on the patent committee. The Company has determined that the options to extend the term of the agreement and the options to license product candidates, including those related to Celgene’s opt-in right for a declined product candidate, are substantive options. Celgene is not contractually obligated to exercise the options. Additionally, as a result of the uncertain outcome of the discovery, research and development activities, the Company is at risk with regard to whether Celgene will exercise the options. Moreover, the Company has determined that the options are not priced at a significant and incremental discount. Accordingly, the options are not considered deliverables at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration. The Company has determined that the potential obligation to manufacture or have manufactured supplies of vectors and associated payloads for incorporation into an optioned product candidate is contingent upon Celgene exercising its option to license a product candidate resulting from the collaboration. Therefore, consistent with the treatment of the options to license product candidates, the Company’s potential obligation under a manufacturing and supply agreement is not considered a deliverable at the inception of the arrangement and the associated fees are not included in allocable arrangement consideration.
	The Company concluded that each of the three deliverables identified at the inception of the arrangement (discovery, research and development services, participation on the JSC and participation on the patent committee) has standalone value from the other undelivered elements. Additionally, the Collaboration Agreement does not include return rights related to the initial collaboration term. Accordingly, each deliverable qualifies as a separate unit of accounting.
	The Company identified the allocable arrangement consideration as the $75,000 up-front payment. The Company determined that each of the identified deliverables have the same period of performance (the three year initial term) and have the same pattern of revenue recognition, ratably over the period of performance. As a result, the $75,000 arrangement consideration will be recognized over the three year initial term.
	The Company evaluated all of the milestones that may be received in connection with Celgene’s option to license a product candidate resulting from the collaboration. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. All clinical and regulatory milestones are considered substantive on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the milestone as well as the level of effort and investment required. Accordingly, such amounts will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met.
	During the three and six months ended June 30, 2014, the Company recognized $6,250 and $12,500, respectively, of revenue associated with its collaboration with Celgene related to the recognition of discovery, research and development services. As of June 30, 2014, there was $42,708 of deferred revenue related to the Company’s collaboration with Celgene which is classified as current or non-current in the condensed consolidated balance sheets based on the contractual term of the arrangement.
	Association Française contre les Myopathies
	In January 2011, the Company entered into a research funding agreement with the Association Française contre les Myopathies (“AFM”), a nonprofit organization dedicated to curing rare neuromuscular diseases and providing treatments to reduce the associated disabilities of such diseases. As part of the agreement, AFM funded the Company 1,000 Euros to be used to advance the Company’s research, process development, manufacturing, preclinical development, and clinical development in gene therapy for beta-hemoglobinopathies in ß-thalassemia and/or in Sickle Cell Disease.
	The funding, or a portion thereof depending on timing, shall be repaid to AFM upon any of the following events: (i) upon out-licensing or sale of the program, (ii) upon obtaining the first product authorization for the market, or (iii) upon sale of the Company, provided that development of the program is active at the time of such sale. The agreement is for a period of four years. The Company believes that repayment of the funds paid under the agreement is not probable as of the date of the agreement, June 30, 2014 or December 31, 2013. The Company recognizes the revenue under this arrangement on a straight-line basis over the term of the agreement and will reassess the probability of repayment at the end of each reporting period.

Business_combinations

Business combinations	6 Months Ended
	Jun. 30, 2014
Business Combinations [Abstract]	'
Business combinations	'
	6. Business combinations
	On June 30, 2014, the Company completed its acquisition of Pregenen, a privately-held biotechnology company, upon which Pregenen became a wholly-owned subsidiary. As a result, the Company obtained gene editing and cell signaling technology with a broad range of potential therapeutic applications.
	Under the terms of the Stock Purchase Agreement, the Company purchased all of Pregenen’s outstanding capital stock in exchange for 405 unregistered shares of common stock and $5,093 in cash. The consideration for the transaction also includes an additional 94 shares of common stock that will be held for a period of 18 months after the acquisition and may be used to settle certain claims for indemnification for breaches or inaccuracies in Pregenen’s representations and warranties, covenants, and agreements and an additional 2 shares relating to a working capital adjustment. The Stock Purchase Agreement also provides for up to $135,000 in future contingent cash payments upon the achievement of certain preclinical, clinical and commercial milestones related to the Pregenen technology, of which $15,000 relates to preclinical milestones, $20,100 relates to clinical milestones and $99,900 relates to commercial milestones.
	The acquisition-date fair value of the purchase consideration is as follows:
	Cash	$	5,093
	Common stock		19,348
	Contingent consideration		6,550
	Total purchase consideration	$	30,991
	Common stock in the table above is comprised of $15,636 in common stock transferred, $3,630 in common stock that will be held back for a period of 18 months and $82 related to a working capital adjustment.
	The transaction was accounted for as a business combination under the acquisition method of accounting. Accordingly, the tangible and identifiable intangible assets acquired and liabilities assumed were recorded at estimated fair value as of the date of acquisition, with the remaining consideration transferred recorded as goodwill.
	The following table summarizes the estimated fair value of the assets acquired and liabilities assumed at the date of acquisition:
		June 30, 2014
	Cash	$	420
	Gene editing platform intangible asset		30,100
	Goodwill		13,128
	Other assets acquired		111
	Total assets acquired		43,759
	Deferred tax liabilities		11,797
	Other liabilities assumed		971
	Total liabilities assumed		12,768
	Net assets acquired	$	30,991
	The purchase price allocation has been prepared on a preliminary basis and is subject to change as additional information becomes available concerning the fair value and tax basis of the assets acquired and liabilities assumed and contingent consideration. Any adjustments to the purchase price allocation will be made as soon as practicable, but no later than one year from June 30, 2014, the acquisition date.
	The fair value of the gene editing platform intangible asset was determined using a relief from royalty approach, including assumptions of projected revenues and royalty rate in addition to a discount rate of 15.5% applied to the projected cash flows. The Company considers the intangible asset acquired to be developed technology, as at the date of the acquisition it could be used the way it is intended to be used in certain ongoing research and development activities. The Company believes the assumptions are representative of those a market participant would use in estimating fair value. The gene editing platform intangible asset will be amortized to research and development expense over its expected useful life of approximately eight years.
	The estimated amortization of intangible assets for the year ended December 31, 2014 and for each of the five succeeding years and thereafter is as follows:
	2014	$	1,881
	2015		3,763
	2016		3,763
	2017		3,763
	2018		3,763
	2019 and thereafter		13,167
	Total	$	30,100
	The deferred tax liabilities of $11,797 primarily relate to the tax impact of future amortization or impairments associated with the identified intangible asset, which is not deductible for tax purposes. See Note 8 “Income taxes,” for additional information.
	Goodwill is calculated as the difference between the acquisition-date fair value of the consideration transferred and the fair values of the assets acquired and liabilities assumed and is not expected to be deductible for income tax purposes. Goodwill is recorded as an indefinite-lived asset and is not amortized but tested for impairment on an annual basis or when indications of impairment exist. Among the factors which resulted in goodwill for the Pregenen acquisition was the opportunity to recognize synergies with the Company’s existing gene insertion platform and deferred tax liabilities recognized in connection with the acquisition.
	The Company incurred a total of $171 in transaction costs in connection with the acquisition, which were included in general and administrative expenses within the condensed consolidated statements of operations and comprehensive loss for the three and six months ended June 30, 2014.
	In connection with the acquisition, the Company issued 3 shares of common stock to a former consultant of Pregenen and recognized $151 of expense within general and administrative expenses in the condensed consolidated statements of operations and comprehensive loss for the three and six months ended June 30, 2014.

Stockbased_compensation_and_wa

Stock-based compensation and warrants	6 Months Ended
	Jun. 30, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Stock-based compensation and warrants	'
	7. Stock-based compensation and warrants
	In 2013, the Company’s board of directors and stockholders approved the 2013 Stock Option and Incentive Plan (“2013 Plan”) which allows for the granting of incentive stock options, non-qualified stock options, restricted stock and restricted stock unit awards, as well as other equity awards to the employees, members of the board of directors and consultants of the Company. The 2013 Plan replaced the 2010 Stock Option and Grant Plan. In January 2014, the number of shares of common stock available for issuance under the 2013 Plan was increased by 960 shares as a result of the automatic increase provision of the 2013 Plan. As of June 30, 2014, the total number of shares of common stock available for issuance under the 2013 Plan was 806.
	Stock-based compensation expense
	Stock-based compensation expense by award type was as follows:
		Three months					Six months ended
		ended June 30,					June 30,
		2014			2013		2014		2013
	Stock options	$	2,335		$	1,623	$	4,812	$	2,262
	Restricted stock awards		16			34		31		56
		$	2,351		$	1,657	$	4,843	$	2,318
	As of June 30, 2014, there was $21,288 of unrecognized stock-based compensation expense related to unvested stock options and restricted stock awards that is expected to be recognized over a weighted-average period of 3.0 years.
	During the six months ended June 30, 2014, the Company issued 2 shares of restricted common stock in exchange for consulting services. The shares vested upon issuance and the Company recognized $42 of expense related to the services provided.
	During the six months ended June 30, 2014, 123 options held by former employees were modified to extend the expiration date. The modification was valued using a Black-Scholes option valuation model and the Company accounted for the $643 of incremental value within general and administrative expenses.
	Restricted common stock
		Shares			Weighted-average
					grant date
					fair value
	Unvested balance at December 31, 2013		69		$	0.95
	Granted		—			—
	Vested		(42	)		0.95
	Forfeited		—			—
	Unvested balance at June 30, 2014		27		$	0.95
	Stock options
	The following table summarizes the stock option activity under the Company’s equity award plans:
		Shares			Weighted-
					average
					exercise
					price
					per share
	Outstanding at December 31, 2013		3,958		$	5.21
	Granted		937		$	23.66
	Exercised		(691	)	$	2.9
	Canceled or forfeited		(92	)	$	14.98
	Outstanding at June 30, 2014		4,112		$	9.59
	Exercisable at June 30, 2014		1,383		$	3.61
	Vested and expected to vest at June 30, 2014		4,058		$	9.64
	Options exercisable for 691 shares of common stock were exercised during the six months ended June 30, 2014, resulting in total proceeds to the Company of $1,864. In accordance with the stock option plans, the shares were issued from a pool of shares reserved for issuance under the stock option plans described above.
	Employee stock purchase plan
	On June 3, 2013, the Company’s board of directors adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which was subsequently approved by its stockholders and became effective upon the closing of the Company’s initial public offering on June 24, 2013. The 2013 ESPP authorizes the initial issuance of up to a total of 238 shares of the Company’s common stock to participating employees. The first offering period under the 2013 ESPP opened on August 1, 2014.
	Warrants
	As of June 30, 2014 and December 31, 2013, the Company had 338 warrants outstanding to purchase common stock. During the three and six months ended June 30, 2014, there were no warrants exercised and no cancellations or expirations.

Income_taxes

Income taxes	6 Months Ended
	Jun. 30, 2014
Income Tax Disclosure [Abstract]	'
Income taxes	'
	8. Income taxes
	Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. The Company has allocated its valuation allowance in accordance with the provisions of ASC 740, Income Taxes, which resulted in a current deferred tax asset of $3,106 and a non-current deferred tax liability of $3,106 as of June 30, 2014.
	As a result of the acquisition of Pregenen, the Company recorded intangible assets for which there is no tax basis, resulting in a deferred tax liability. The deferred tax liabilities can be used as a source of income to realize the Company’s pre-existing deferred tax assets, thereby resulting in a decrease to the corresponding valuation allowance. The decrease in the valuation allowance resulted in an $11,797 income tax benefit for the three and six months ended June 30, 2014.

Net_loss_per_share

Net loss per share	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Net loss per share	'
	9. Net loss per share
	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect:
		Three months				Six months
		ended June 30,				ended June 30,
		2014		2013		2014		2013
	Warrants		338		440		338		440
	Outstanding stock options		4,112		3,890		4,112		3,890
	Unvested restricted stock		27		110		27		110
	Acquisition holdback (Note 6)		94		—		94		—
			4,571		4,440		4,571		4,440

Subsequent_events

Subsequent events	6 Months Ended
	Jun. 30, 2014
Subsequent Events [Abstract]	'
Subsequent events	'
	10. Subsequent events
	In July 2014, the Company sold 3,450 shares of common stock (inclusive of 450 shares of common stock sold by the Company pursuant to the full exercise of an overallotment option granted to the underwriters in connection with the offering) through an underwritten public offering at a price of $34.00 per share. The aggregate net proceeds received by the Company from the offering were $109,776, net of underwriting discounts and commissions and estimated offering expenses payable by the Company of approximately $7,524.
	In connection with the Pregenen acquisition, the number of shares comprising the purchase price consideration was increased from 499 to 501 due to a working capital adjustment completed on July 29, 2014. On July 30, 2014, the Company registered a total of 505 shares of common stock issued or to be issued to the former equityholders of Pregenen under the Securities Act of 1933.

Summary_of_significant_account1

Summary of significant accounting policies and basis of presentation (Policies)	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Basis of presentation and principles of consolidation	'
	Basis of presentation and principles of consolidation
	The accompanying condensed consolidated financial statements are unaudited and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”) as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended June 30, 2014 and 2013.
	The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on March 5, 2014.
	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Precision Genome Engineering, Inc., bluebird bio France – SARL, bluebird bio Australia Pty Ltd. and bluebird bio Securities Corporation. All intercompany balances and transactions have been eliminated in consolidation. The assets acquired and liabilities assumed in connection with the Company’s acquisition of Pregenen were recorded at their fair values as of June 30, 2014, the date of the acquisition, and the operating results of Pregenen have been consolidated with those of the Company from the date of acquisition. Any reference in these notes to applicable guidance is meant to refer to GAAP. The Company views its operations and manages its business in one operating segment. All material long-lived assets of the Company reside in the United States.
Summary of accounting policies	'
	Summary of accounting policies
	The significant accounting policies described in the Company’s audited financial statements as of and for the year ended December 31, 2013, and the notes thereto, which are included in the Annual Report on Form 10-K, have had no material changes during the six months ended June 30, 2014, except as noted below:
Business combinations	'
	Business combinations
	On June 30, 2014, the Company completed its acquisition of Pregenen for total consideration of $30,991, consisting of cash consideration of $5,093, common stock consideration of $19,348 and contingent consideration with an estimated fair value of $6,550. The estimated fair value of the contingent consideration is based upon significant assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and discount rates. The estimated fair value could materially differ from actual values or fair values determined using different assumptions. See Note 3, “Fair value measurements,” for additional information.
	This transaction was accounted for as a business combination under the acquisition method of accounting. Accordingly, the tangible assets and identifiable intangible assets acquired and liabilities assumed were recorded at fair value as of the date of acquisition, with the remaining purchase price recorded as goodwill. The estimated fair values of the assets acquired and liabilities assumed at the date of acquisition are summarized in Note 6, “Business combinations.” The estimated fair values of acquired assets and assumed liabilities were determined using the methods discussed in the following paragraphs and require significant judgment and estimates, which could materially differ from actual values and fair values determined using different methods or assumptions.
	The purchase price allocation was initially prepared on a preliminary basis and is subject to change as additional information becomes available concerning the fair value and tax basis of the assets acquired and liabilities assumed and contingent consideration. Any adjustments to the purchase price allocations are made as soon as practicable but no later than one year from the acquisition date.
Goodwill	'
	Goodwill
	Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting for business combinations. Goodwill is not amortized but is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company has not recognized any impairment charges related to goodwill.
Intangible assets	'
	Intangible assets
	Intangible assets consist of acquired core technology with finite lives. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives. The Company evaluates the potential impairment of intangible assets if events or changes in circumstances indicate that the carrying amount of the assets may not be fully recoverable or that the useful lives of these assets are no longer appropriate. The impairment test is based on a comparison of the undiscounted cash flows expected to be generated from the use of the asset group and its eventual disposition to the carrying value of the asset group. If impairment is indicated, the asset is written down by the amount by which the carrying value of the asset exceeds the related fair value of the asset. The Company has not recognized an impairment charge related to intangible assets.
Contingent consideration	'
	Contingent consideration
	Each reporting period, the Company revalues the contingent consideration obligations associated with business combinations to their fair value and record within operating expenses increases in their fair value as contingent consideration expense and decreases in the fair value as contingent consideration income. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones are achieved and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s clinical programs in certain indications progress and additional data are obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 3, “Fair value measurements,” for additional information.
Use of estimates	'
	Use of estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates are used in the following areas, among others: acquisition-date fair value and subsequent fair value estimates used to assess impairment of long-lived assets, including goodwill and intangible assets, contingent consideration, stock-based compensation expense, accrued expenses, revenue and income taxes.
Fair value of financial instruments	'
	Fair value of financial instruments
	The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC 820, Fair Value Measurements and Disclosures, establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:
	Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.
	Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly.
	Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.
	The carrying amounts of accounts payable and accrued expenses approximate their fair values due to their short-term maturities.
Other Income and Expense	'
	Other Income and Expense
	In March 2014, the Company received an award of $306 of tax incentives from the Massachusetts Life Sciences Center, which will allow the Company to monetize approximately $276 of state research and development tax credits. In exchange for these incentives, the Company pledged to hire an incremental fifteen employees and to maintain the additional headcount through at least December 31, 2018. Failure to do so could result in the Company being required to repay some or all of these incentives. As the Company met the additional headcount during the quarter ended June 30, 2014, the Company has deferred and is amortizing the benefit of this monetization on a straight-line basis over the five-year performance period.
Net Income (Loss) Per Share	'
	Net Income (Loss) Per Share
	Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including any dilutive effect from outstanding stock options, unvested restricted stock, warrants, and acquisition holdback shares using the treasury stock method.
Recent accounting pronouncements	'
	Recent accounting pronouncements
	During the quarter ended June 30, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU No. 2014-09”), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective on January 1, 2017 and early adoption is not permitted for public entities. The Company is currently evaluating the potential impact that ASU No. 2014-09 may have on its financial position and results of operations.

Fair_value_measurements_Tables

Fair value measurements (Tables)	6 Months Ended
	Jun. 30, 2014
Fair Value Disclosures [Abstract]	'
Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis	'
	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2014 and December 31, 2013:
	Description	Total		Quoted		Significant		Significant
				prices in		other		unobservable
				active		observable		inputs
				markets		inputs		(Level 3)
				(Level 1)		(Level 2)
	30-Jun-14
	Assets:
	Cash and cash equivalents	$	175,710	$	175,710	$	—	$	—
	Total assets	$	175,710	$	175,710	$	—	$	—
	Liabilities:
	Contingent consideration	$	6,550	$	—	$	—	$	6,550
	Total liabilities	$	6,550	$	—	$	—	$	6,550
	31-Dec-13
	Total cash and cash equivalents	$	206,279	$	206,279	$	—	$	—
Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations	'
	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations which include Level 3 inputs:
		Six months ended
		June 30, 2014
	Beginning balance	$	—
	Additions		6,550
	Changes in fair value		—
	Payments		—
	Ending balance	$	6,550

Business_combinations_Tables

Business combinations (Tables)	6 Months Ended
	Jun. 30, 2014
Business Combinations [Abstract]	'
Schedule of Acquisition-Date Fair Value	'
	The acquisition-date fair value of the purchase consideration is as follows:
	Cash	$	5,093
	Common stock		19,348
	Contingent consideration		6,550
	Total purchase consideration	$	30,991
Estimated Fair Value of Assets Acquired and Liabilities Assumed	'
	The following table summarizes the estimated fair value of the assets acquired and liabilities assumed at the date of acquisition:
		June 30, 2014
	Cash	$	420
	Gene editing platform intangible asset		30,100
	Goodwill		13,128
	Other assets acquired		111
	Total assets acquired		43,759
	Deferred tax liabilities		11,797
	Other liabilities assumed		971
	Total liabilities assumed		12,768
	Net assets acquired	$	30,991
Estimated Amortization of Intangible Assets	'
	The estimated amortization of intangible assets for the year ended December 31, 2014 and for each of the five succeeding years and thereafter is as follows:
	2014	$	1,881
	2015		3,763
	2016		3,763
	2017		3,763
	2018		3,763
	2019 and thereafter		13,167
	Total	$	30,100

Stockbased_compensation_and_wa1

Stock-based compensation and warrants (Tables)	6 Months Ended
	Jun. 30, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]	'
Summary of Stock-Based Compensation Expense by Award Type	'
	Stock-based compensation expense by award type was as follows:
		Three months					Six months ended
		ended June 30,					June 30,
		2014			2013		2014		2013
	Stock options	$	2,335		$	1,623	$	4,812	$	2,262
	Restricted stock awards		16			34		31		56
		$	2,351		$	1,657	$	4,843	$	2,318
Summary of Restricted Common Stock	'
	Restricted common stock
		Shares			Weighted-average
					grant date
					fair value
	Unvested balance at December 31, 2013		69		$	0.95
	Granted		—			—
	Vested		(42	)		0.95
	Forfeited		—			—
	Unvested balance at June 30, 2014		27		$	0.95
Summary of Stock Option Activity Under Plan	'
	The following table summarizes the stock option activity under the Company’s equity award plans:
		Shares			Weighted-
					average
					exercise
					price
					per share
	Outstanding at December 31, 2013		3,958		$	5.21
	Granted		937		$	23.66
	Exercised		(691	)	$	2.9
	Canceled or forfeited		(92	)	$	14.98
	Outstanding at June 30, 2014		4,112		$	9.59
	Exercisable at June 30, 2014		1,383		$	3.61
	Vested and expected to vest at June 30, 2014		4,058		$	9.64

Net_loss_per_share_Tables

Net loss per share (Tables)	6 Months Ended
	Jun. 30, 2014
Earnings Per Share [Abstract]	'
Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	'
	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect:
		Three months				Six months
		ended June 30,				ended June 30,
		2014		2013		2014		2013
	Warrants		338		440		338		440
	Outstanding stock options		4,112		3,890		4,112		3,890
	Unvested restricted stock		27		110		27		110
	Acquisition holdback (Note 6)		94		—		94		—
			4,571		4,440		4,571		4,440

Summary_of_significant_account2

Summary of significant accounting policies and basis of presentation - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Jun. 30, 2014
		Employees
		Segment
Summary Of Basis Of Presentation And Significant Accounting Policies [Line Items]	'	'
Number of operating segment	'	1
Contingent consideration	'	$6,550
Tax incentives	306	'
New employees hired to maintain additional headcount	'	15
Pregenen [Member]	'	'
Summary Of Basis Of Presentation And Significant Accounting Policies [Line Items]	'	'
Total consideration	'	30,991
Cash consideration	'	5,093
Common stock consideration	'	19,348
Contingent consideration	'	6,550
State Research and Development [Member]	'	'
Summary Of Basis Of Presentation And Significant Accounting Policies [Line Items]	'	'
Tax credits	$276	'
Deferred benefit amortization period	'	'5 years

Fair_value_measurements_Record

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'	'
Total cash and cash equivalents	$175,710	$206,279
Assets:	'	'
Cash and cash equivalents	175,710	206,279
Total assets	175,710	'
Liabilities:	'	'
Contingent consideration	6,550	'
Total liabilities	6,550	'
Quoted prices in active markets (Level 1) [Member]	'	'
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'	'
Total cash and cash equivalents	175,710	206,279
Assets:	'	'
Cash and cash equivalents	175,710	206,279
Total assets	175,710	'
Liabilities:	'	'
Contingent consideration	'	'
Total liabilities	'	'
Significant other observable inputs (Level 2) [Member]	'	'
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'	'
Total cash and cash equivalents	'	'
Assets:	'	'
Cash and cash equivalents	'	'
Total assets	'	'
Liabilities:	'	'
Contingent consideration	'	'
Total liabilities	'	'
Significant unobservable inputs (Level 3) [Member]	'	'
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'	'
Total cash and cash equivalents	'	'
Assets:	'	'
Cash and cash equivalents	'	'
Total assets	'	'
Liabilities:	'	'
Contingent consideration	6,550	'
Total liabilities	$6,550	'

Fair_value_measurements_Additi

Fair value measurements - Additional Information (Detail) (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'
Cash equivalents maturities	'Three months or less
Accrued Expenses and Other Current Liabilities [Member]	'
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'
Fair value of contingent consideration obligations	453
Minimum [Member]	'
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'
Milestone achievement period	'2015
Milestone discount rates	10.00%
Maximum [Member]	'
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]	'
Milestone achievement period	'2026
Milestone discount rates	15.00%

Fair_value_measurements_RollFo

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) (Contingent consideration obligations [Member], USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014
Contingent consideration obligations [Member]	'
Fair Value Assets Measured On Recurring and Nonrecurring Basis [Line Items]	'
Beginning balance	'
Additions	6,550
Changes in fair value	'
Payments	'
Ending balance	$6,550

Commitments_and_contingencies_

Commitments and contingencies - Additional Information (Detail) (USD $)

0 Months Ended

6 Months Ended

6 Months Ended

36 Months Ended

0 Months Ended

In Thousands, unless otherwise specified

Jun. 09, 2014

Jun. 03, 2013

Jun. 30, 2014

Jun. 09, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2012

Dec. 31, 2013

Jun. 03, 2013

Jun. 03, 2013

Jun. 03, 2013

sqft

sqft

Pregenen [Member]

Pregenen [Member]

Pregenen [Member]

Pregenen [Member]

French Credit d'Impot Recherche Program [Member]

French Credit d'Impot Recherche Program [Member]

French Credit d'Impot Recherche Program [Member]

Rent Commencement Date [Member]

First Anniversary of the Rent Commencement Date [Member]

Second Anniversary of the Rent Commencement Date [Member]

Preclinical milestones [Member]

Clinical Milestone Payments [Member]

Commercial Milestones Payments [Member]

Commitments And Contingencies Disclosure [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Lease period

'

'9 years

'

'

'

'

'

'

'

'

'

'

'

'

Lease building space

'

43,600

'

'

'

'

'

'

'

'

'

'

'

'

Monthly lease payments

'

$209

'

'

'

'

'

'

'

'

'

'

'

'

Rent abatement

'

209

'

'

'

'

'

'

'

'

'

'

'

'

Contribution from the landlord towards the initial build-out of the space

1,234

6,538

'

'

'

'

'

'

'

'

'

'

'

'

Option to extend this lease

'

'5 years

'

'

'

'

'

'

'

'

'

'

'

'

Cash-collateralized irrevocable standby letter of credit

'

1,253

'

'

'

'

'

'

'

'

'

1,044

835

627

Lease expiration date

'

'

31-Mar-15

'

'

'

'

'

'

'

'

'

'

'

Additional lease building space

9,869

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional monthly lease payments

'

'

'

47

'

'

'

'

'

'

'

'

'

'

Amendment effective date

'2014-09

'

'

'

'

'

'

'

'

'

'

'

'

'

Eligible percentage of research expense monetized

'

'

'

'

'

'

'

'

30.00%

'

'

'

'

'

Aggregate reimbursement

'

'

'

'

'

'

'

'

'

807

'

'

'

'

Current assets

'

'

'

'

'

'

'

'

'

'

873

'

'

'

Non-current assets

'

'

'

'

'

'

'

'

566

'

'

'

'

'

Additional future contingent cash payments

'

'

'

'

$135,000

$15,000

$20,100

$99,900

'

'

'

'

'

'

Significant_agreements_Additio

Significant agreements - Additional Information (Detail)

0 Months Ended

3 Months Ended

6 Months Ended

0 Months Ended

1 Months Ended

In Thousands, unless otherwise specified

Mar. 19, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Mar. 19, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jan. 31, 2011

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Celgene Corporation [Member]

Association Francaise Contre Les Myopathies [Member]

Collaborative Arrangement [Member]

Collaborative Arrangement [Member]

Collaborative Arrangement [Member]

Collaborative Arrangement [Member]

Collaborative Arrangement [Member]

Collaborative Arrangement [Member]

Collaborative Arrangement, Product [Member]

Collaborative Arrangement, Product [Member]

Collaborative Arrangement, Product [Member]

Collaborative Arrangement, Product [Member]

Collaborative Arrangement, Co-promotion and Development [Member]

Collaborative Arrangement, Co-promotion and Development [Member]

Collaborative Arrangement, Co-promotion and Development [Member]

Collaborative Arrangement, Co-promotion and Development [Member]

Research and Development Arrangement [Member]

USD ($)

USD ($)

Maximum [Member]

Minimum [Member]

Up-front Payment Arrangement [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

EUR (€)

Deliverables

Option

USD ($)

Option Fees [Member]

Clinical Milestone Payments [Member]

Regulatory Milestone Payments [Member]

Commercial Milestones Payments [Member]

Option Fees [Member]

Clinical Milestone Payments [Member]

Regulatory Milestone Payments [Member]

Commercial Milestones Payments [Member]

Deliverables

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Collaboration agreement, cash payment received

'

'

'

'

'

$75,000

'

'

'

'

'

'

'

'

'

Term of collaboration agreement

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of extension options

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

Maturity date of extension option

'

'

'

19-Mar-19

'

'

'

'

'

'

'

'

'

'

'

Amount per product eligible to be received upon achievement of specified event

'

'

'

'

'

'

20,000

10,000

117,000

78,000

10,000

10,000

54,000

36,000

'

Percentage of Change in control transaction which provides for the right to terminate agreement

'

'

'

'

50.00%

'

'

'

'

'

'

'

'

'

'

Collaboration period

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

Extended options period

'

'

'2 years

'

'

'

'

'

'

'

'

'

'

'

'

Number of deliverable

'

3

3

'

'

'

'

'

'

'

'

'

'

'

'

Deferred revenue recognition period

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

Deferred revenue recognized

'

6,250

12,500

'

'

'

'

'

'

'

'

'

'

'

'

Deferred revenue

'

$42,708

$42,708

'

'

'

'

'

'

'

'

'

'

'

€ 1,000

Term of research funding agreement

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'4 years

Business_Combinations_Addition

Business Combinations - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2014
Business Acquisition [Line Items]	'	'
Expenses related to consulting service	'	$168
Pregenen [Member]	'	'
Business Acquisition [Line Items]	'	'
Acquisition completion date	'	30-Jun-14
Shares of the company common stock	'	405
Cash paid to purchase of common stock	'	5,093
Common stock holding period after acquisition	'	'18 months
Contingent cash payments	135,000	135,000
Consideration transferred to acquisition	19,348	19,348
Consideration transferred to acquisition, held back period	'	'18 months
Business combination working capital adjustment	'	82
Adjustments to purchase price allocation period	'	'1 year
Discount rate applied to projected cash flows	'	15.50%
Gene editing platform expected useful life	'	'8 years
Deferred tax liabilities related to intangible asset	11,797	11,797
Transaction costs	171	171
Pregenen [Member] | Preclinical milestones [Member]	'	'
Business Acquisition [Line Items]	'	'
Contingent cash payments	15,000	15,000
Pregenen [Member] | Clinical milestones [Member]	'	'
Business Acquisition [Line Items]	'	'
Contingent cash payments	20,100	20,100
Pregenen [Member] | Commercial Milestones Payments [Member]	'	'
Business Acquisition [Line Items]	'	'
Contingent cash payments	99,900	99,900
Additional shares relating to working capital adjustment	'	2
Pregenen [Member] | Consultant [Member]	'	'
Business Acquisition [Line Items]	'	'
Issuance of stock in exchange of consulting services	3	3
Expenses related to consulting service	151	151
Pregenen [Member] | Upfront Payment [Member]	'	'
Business Acquisition [Line Items]	'	'
Consideration transferred to acquisition	15,636	15,636
Pregenen [Member] | Held back period of 18 months [Member]	'	'
Business Acquisition [Line Items]	'	'
Consideration transferred to acquisition	$3,630	$3,630
Pregenen [Member] | Additional shares of common stock [Member]	'	'
Business Acquisition [Line Items]	'	'
Shares of the company common stock	'	94

Business_Combinations_Schedule

Business Combinations - Schedule of Acquisition-Date Fair Value (Detail) (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014
Business Acquisition [Line Items]	'
Contingent consideration	$6,550
Pregenen [Member]	'
Business Acquisition [Line Items]	'
Cash	5,093
Common stock	19,348
Contingent consideration	6,550
Total purchase consideration	$30,991

Business_combinations_Estimate

Business combinations - Estimated Fair Value of Assets Acquired and Liabilities Assumed (Detail) (USD $)	Jun. 30, 2014
In Thousands, unless otherwise specified
Business Acquisition [Line Items]	'
Gene editing platform intangible asset	$30,100
Goodwill	13,128
Pregenen [Member]	'
Business Acquisition [Line Items]	'
Cash	420
Gene editing platform intangible asset	30,100
Goodwill	13,128
Other assets acquired	111
Total assets acquired	43,759
Deferred tax liabilities	11,797
Other liabilities assumed	971
Total liabilities assumed	12,768
Net assets acquired	$30,991

Business_Combinations_Schedule1

Business Combinations - Schedule of Estimated Amortization of Intangible Assets (Detail) (USD $)	Jun. 30, 2014
In Thousands, unless otherwise specified
Finite Lived Intangible Assets Future Amortization Expense [Abstract]	'
2014	$1,881
2015	3,763
2016	3,763
2017	3,763
2018	3,763
2019 and thereafter	13,167
Total	$30,100

Stockbased_compensation_and_wa2

Stock-based compensation and warrants - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended
In Thousands, unless otherwise specified	Jan. 31, 2014	Jun. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Increased number of issuance of awards under the 2013 Plan	960	'	'	'
Number of shares available for issuance	'	806	806	'
Unrecognized stock- based compensation expense related to unvested stock options and restricted stock awards	'	$21,288	$21,288	'
Expected period for stock option	'	'	'3 years	'
Expenses related to consulting service	'	'	168	'
Stock options held by former employees	'	123	123	'
Incremental value on option valuation	'	'	643	'
Stock option share exercised	'	'	691	'
Proceed from option share exercised	'	'	1,864	'
Warrants outstanding	'	338	338	338
Warrants exercised	'	0	0	'
Warrants cancelled or expired	'	0	0	'
Restricted Common Stock [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Issuance of stock in exchange of consulting services	'	'	2	'
Expenses related to consulting service	'	'	$42	'
Employee Stock Purchase Plan [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Initial issuance of common stock	'	238	238	'

Stockbased_compensation_and_wa3

Stock-based compensation and warrants - Summary of Stock-Based Compensation Expense by Award Type (Detail) (USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	$2,351	$1,657	$4,843	$2,318
Stock options [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	2,335	1,623	4,812	2,262
Restricted Stock [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based compensation expense	$16	$34	$31	$56

Stockbased_compensation_and_wa4

Stock-based compensation and warrants - Summary of Restricted Common Stock (Detail) (Restricted Common Stock [Member], USD $)	6 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2014
Restricted Common Stock [Member]	'
Shares	'
Unvested balance at beginning of period	69
Granted	'
Vested	-42
Forfeited	'
Unvested balance at end of period	27
Weighted-average grant date fair value	'
Unvested balance at beginning of period	$0.95
Granted	'
Vested	$0.95
Forfeited	'
Unvested balance at end of period	$0.95

Stockbased_compensation_and_wa5

Stock-based compensation and warrants - Summary of Stock Option Activity Under Plan (Detail) (USD $)	6 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2014
Shares	'
Outstanding at beginning of period	3,958
Granted	937
Exercised	-691
Canceled or forfeited	-92
Outstanding at end of period	4,112
Exercisable at end of period	1,383
Vested and expected to vest at end of period	4,058
Weighted-average exercise price per share	'
Outstanding at beginning of period	$5.21
Granted	$23.66
Exercised	$2.90
Canceled or forfeited	$14.98
Outstanding at end of period	$9.59
Exercisable at end of period	$3.61
Vested and expected to vest at end of period	$9.64

Income_taxes_Additional_Inform

Income taxes - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'	'	'
Current deferred tax asset	$3,106	$3,106	$693
Non-current deferred tax liability	3,106	3,106	693
Income tax benefit	$11,797	$11,797	'

Net_loss_per_share_Common_Stoc

Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Common stock equivalents excluded from the calculation of diluted net loss per share	4,571	4,440	4,571	4,440
Warrants [Member]	'	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Common stock equivalents excluded from the calculation of diluted net loss per share	338	440	338	440
Stock options [Member]	'	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Common stock equivalents excluded from the calculation of diluted net loss per share	4,112	3,890	4,112	3,890
Restricted Stock [Member]	'	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Common stock equivalents excluded from the calculation of diluted net loss per share	27	110	27	110
Acquisition holdback [Member]	'	'	'	'
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Common stock equivalents excluded from the calculation of diluted net loss per share	94	'	94	'

Subsequent_events_Additional_I

Subsequent events - Additional Information (Detail) (Subsequent Event [Member], USD $)	1 Months Ended		0 Months Ended			1 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jul. 31, 2014	Jul. 31, 2014	Jul. 30, 2014	Jul. 29, 2014	Jul. 29, 2014	Jul. 31, 2014
		Common Stock [Member]	Common Stock [Member]	Common Stock [Member]	Common Stock [Member]	Overallotment Option Granted to Underwriters [Member]
			Pregenen [Member]	Minimum [Member]	Maximum [Member]	Common Stock [Member]
				Pregenen [Member]	Pregenen [Member]
Subsequent Event [Line Items]	'	'	'	'	'	'
Shares of common stock sold	'	3,450	'	'	'	450
Shares of common stock sold, price per share	'	$34	'	'	'	'
Aggregate net proceeds received from the offering	$109,776	'	'	'	'	'
Offering expenses payable	$7,524	'	'	'	'	'
Business acquisition purchase price consideration	'	'	'	499	501	'
Common stock issued	'	'	505	'	'	'