Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2013

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, 2012	5,349,810		$	5.98	7.0
Granted	38,750		$	13.01
Exercised	(370,627	)	$	6.17
Canceled	(12,330	)	$	6.64
Options Outstanding at June 30, 2013	5,005,603		$	6.02	6.5
Options Vested and Expected to Vest at June 30, 2013	4,967,369		$	6.02	6.5
Options Exercisable at June 30, 2013	3,176,119		$	6.58	5.3
Shares Available for Grant under the 2008 Plan	3,313,661

Schedule of stock-based compensation expense

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
	(In thousands)
Research and development	$	422	$	284	$	863	$	679
General and administrative	254		164		521		409
Total stock-based compensation expense	$	676	$	448	$	1,384	$	1,088

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

	Three months ended June 30,				Six months ended June 30,
	2013		2012		2013		2012
Expected stock price volatility	72	%	71	%	72	%	70 - 71	%
Expected option term	6.0 years		6.0 years		6.0 years		6.0 years
Risk-free interest rate	1.2 — 1.8	%	0.9 — 1.3	%	1.2 — 1.8	%	0.9 — 1.4	%
Expected dividend yield	None		None		None		None