Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2013

Stock-Based Compensation

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

	Three months ended March 31, 2013
Risk-free interest rate	1.1%
Expected dividend yield	0%
Expected term	5.8-5.9 years
Expected volatility	67%

Schedule of recognized stock-based compensation expense

	Three months ended March 31,
(in thousands)	2012		2013
Employee awards:
Research and development	$	909	$	1,284
General and administrative	430		1,090
Stock-based compensation for employee awards	1,339		2,374
Stock-based compensation for non-employee awards	—		12
Total stock-based compensation	$	1,339	$	2,386

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	2,719		$	6.35
Exercised	(90	)	$	2.18
Forfeited	(31	)	$	6.76
Outstanding, March 31, 2013	20,664		$	3.87	6.71	$	51,276
Exercisable, March 31, 2013	14,177		$	2.66	5.52	$	49,725
Vested and expected to vest, March 31, 2013	20,154		$	3.80	6.64	$	51,249