Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2012

Stock-Based Compensation

Schedule of recognized stock-based compensation expense

	Years ended December 31,
(in thousands)	2010		2011		2012
Employee awards:
Research and development	$	2,787	$	3,597	$	4,234
General and administrative		1,706		2,875		2,510
Stock-based compensation for employee awards		4,493		6,472		6,744
Stock-based compensation for nonemployee awards		58		480		145
Total stock-based compensation	$	4,551	$	6,952	$	6,889

Schedule of assumptions used to calculate fair value of options granted

	Years ended December 31,
	2010	2011	2012
Risk-free interest rate	1.7 - 2.8%	1.3 - 2.5%	0.7 - 1.1%
Expected dividend yield	0%	0%	0%
Expected term	5 - 5.9 years	5 - 5.9 years	5 - 5.9 years
Expected volatility	73 - 77%	71 - 73%	66 - 72%

Summary of stock option activity

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding at December 31, 2011		17,617		$	2.56
Granted		3,251		$	7.45
Exercised		(2,622	)	$	2.06
Cancelled		(180	)	$	4.34
Outstanding at December 31, 2012		18,066		$	3.50		6.54	$	51,486
Vested and expected to vest at December 31, 2012		17,741		$	3.43		6.49	$	51,434
Exercisable at December 31, 2012		13,616		$	2.52		5.70	$	49,155