Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2014

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, 2013	5,770,544		$	8.17	7.0
Granted	12,750		$	26.37
Exercised	(107,814	)	$	6.10
Canceled	(7,527	)	$	10.46
Options Outstanding at March 31, 2014	5,667,953		$	8.25	6.8
Options Vested and Expected to Vest at March 31, 2014	5,627,995		$	8.21	6.8
Options Exercisable at March 31, 2014	3,371,583		$	6.22	5.5
Shares Available for Grant under the 2008 Plan	2,097,719

Schedule of stock-based compensation expense

	Three months ended March 31,
	2014		2013
	(In thousands)
Research and development	$	591	$	441
General and administrative	659		267
Total stock-based compensation expense	$	1,250	$	708

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

	Three months ended March 31,
	2014		2013
Expected stock price volatility	71	%	72	%
Expected option term	6.0 Years		6.0 Years
Risk-free interest rate	2.2	%	1.4	%
Expected dividend yield	None		None