Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2015

Stock-Based Compensation

Summary of stock option activity

	Shares		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at December 31, 2014	7,015,350		$	9.34	6.6
Granted	70,650		$	25.54
Exercised	(545,867	)	$	5.35
Canceled	(1,213	)	$	13.11
Options Outstanding at March 31, 2015	6,538,920		$	9.85	6.7
Options Vested and Expected to Vest at March 31, 2015	6,506,078		$	9.82	6.7
Options Exercisable at March 31, 2015	3,752,500		$	7.43	5.3
Shares Available for Grant under the 2008 Plan	605,722

Schedule of stock-based compensation expense

	Three months ended March 31,
	2015		2014
	(In thousands)
Research and development	$	1,269	$	591
General and administrative	1,012		659
Total stock-based compensation expense	$	2,281	$	1,250

Schedule of assumptions used for the fair values of employee stock options granted

	Three months ended March 31,
	2015		2014
Expected stock price volatility	69	%	71	%
Expected option term	6.0 Years		6.0 Years
Risk-free interest rate	1.8	%	2.2	%
Expected dividend yield	None		None