Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 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	1,333,850		$	13.57
Exercised	(134,131	)	$	6.05
Canceled	(8,497	)	$	10.51
Options Outstanding at June 30, 2014	6,961,766		$	9.24	7.2
Options Vested and Expected to Vest at June 30, 2014	6,886,817		$	9.19	7.2
Options Exercisable at June 30, 2014	3,480,200		$	6.18	5.3
Shares Available for Grant under the 2008 Plan	765,589

Schedule of stock-based compensation expense

	Three months ended June 30,				Six months ended June 30,
	2014		2013		2014		2013
	(In thousands)
Research and development	$	747	$	422	$	1,338	$	863
General and administrative	764		254		1,423		521
Total stock-based compensation expense	$	1,511	$	676	$	2,761	$	1,384

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

	Three months ended June 30,				Six months ended June 30,
	2014		2013		2014		2013
Expected stock price volatility	72	%	72	%	71 - 72	%	72	%
Expected option term	6.0 years		6.0 years		6.0 years		6.0 years
Risk-free interest rate	2.2	%	1.2 - 1.8	%	2.2	%	1.2 - 1.8	%
Expected dividend yield	None		None		None		None