Share-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2012

Share-Based Compensation [Abstract]

Share-based compensation expense recognized in the accompanying Consolidated Statements of Operations and Comprehensive Loss

Share-based compensation expense for the three and six months ended June 30, 2012 and 2011, respectively, was recognized in the accompanying Consolidated Statements of Operations and Comprehensive Loss as follows:

	Three Months Ended June 30,				Six Months Ended June 30,				Cumulative from April 20, 2009 (Inception) to
	2012		2011		2012		2011		June 30, 2012
Research and development	$	592	$	172	$	1,032	$	206	$	1,693
General and administrative		576		166		1,082		191		1,818
Total share-based compensation expense	$	1,168	$	338	$	2,114	$	397	$	3,511

Summary of stock options activity

The following table summarizes the activity relating to the Company’s options to purchase common stock for the six month period ended June 30, 2012:

	Option Shares Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Balance at December 31, 2011		934,816		$	4.88
Granted		806,698			23.58
Exercised		(13,510	)		3.12
Forfeited		(9,806	)		3.28
Balance at June 30, 2012		1,718,198		$	13.68		9.04	$	15,697,604
Vested and expected to vest at June 30, 2012		1,538,488		$	13.01		8.99	$	14,959,224
Vested at June 30, 2012		304,425		$	1.87		7.88	$	6,031,082

Schedule of share-based compensation arrangements by share-based payment award

The aggregate intrinsic value in the table above represents the pretax intrinsic value, based on our closing stock price of $21.68 as of June 30, 2012, which would have been received by the option holders had all option holders with in-the-money options exercised their options as of that date.

	Three Months Ended June 30,
	2012		2011
Weighted-average grant-date fair value per share	$	12.60	$	9.18
Intrinsic value of options exercised	$	162,845	$	5,423
Cash received from stock option exercises	$	32,108	$	892,221

	Six Months Ended June 30,
	2012		2011
Weighted-average grant-date fair value per share	$	15.02	$	9.24
Intrinsic value of options exercised	$	223,319	$	5,423
Cash received from stock option exercises	$	42,149	$	893,670

Unrecognized share-based compensation expense

As of June 30, 2012, the unrecognized share-based compensation expense related to nonvested options, adjusted for expected forfeitures, and estimated weighted-average remaining vesting period was as follows:

Unrecognized share-based compensation expense             (in millions)	Weighted- average remaining vesting period             (in years)
$                14.6	3.2