Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2013

Stock-Based Compensation

Schedule of assumptions used to calculate fair value of options granted to employees

	Three months ended June 30,		Six months ended June 30,
	2012	2013	2012	2013
Risk-free interest rate	0.9-1.1%	0.1-1.4%	0.9-1.1%	0.1-1.4%
Expected dividend yield	0%	0%	0%	0%
Expected term	5.8-5.9 years	5.3-5.9 years	5.8-5.9 years	5.3-5.9 years
Expected volatility	71-72%	67-68%	71-72%	67-68%

Schedule of recognized stock-based compensation expense

	Three months ended June 30,				Six months ended June 30,
(in thousands)	2012		2013		2012		2013
Employee awards:
Research and development	$	945	$	1,651	$	1,853	$	2,935
General and administrative	432		1,307		862		2,397
Stock-based compensation for employee awards	1,377		2,958		2,715		5,332
Stock-based compensation for non-employee awards	33		72		34		84
Total stock-based compensation	$	1,410	$	3,030	$	2,749	$	5,416

Summary of stock option activity

					Weighted
					Average
	Number		Weighted		Remaining	Aggregate
	of		Average		Contractual	Intrinsic
(in thousands, except per share amounts)	Shares		Exercise Price		Term	Value
Outstanding, December 31, 2012	18,066		$	3.50	6.54	$	51,486
Granted	3,076		$	6.24
Exercised	(499	)	$	2.41
Forfeited	(196	)	$	5.94
Outstanding, June 30, 2013	20,447		$	3.91	6.59	$	60,107
Exercisable, June 30, 2013	14,560		$	2.85	5.53	$	57,350
Vested and expected to vest, June 30, 2013	20,026		$	3.85	6.53	$	59,982