Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2013

Stock-based compensation

Summary of the Company's stock option activity and related information

A summary of the Company’s stock option activity and related information is as follows (in thousands, except share and per share data):

	Shares		Weighted average exercise price per share	Weighted average remaining contractual term (years)	Aggregate intrinsic value
Outstanding at December 31, 2012	1,218,153		3.83	9.32	$	5,535
Granted	437,542		12.78
Exercised	(24,049	)	0.04
Outstanding at March 31, 2013	1,631,646		6.28	9.31	$	6,091
Exercisable at March 31, 2013	227,503		1.55	8.75	1,694
Vested and expected to vest at March 31, 2013	1,487,084		6.22	8.75	$	5,652

Schedule of assumptions used to estimate fair value of each stock-based award on the grant date using the Black-Scholes option pricing model

	March 31, 2013	March 31, 2012
Risk-free interest rate	0.9% - 1.1%	1.2% - 1.84%
Dividend yield	—	—
Volatility	89% - 91%	79% - 82%
Expected term (years)	5.3 — 9.7	6.0 - 9.75

Summary of the status of non-vested RSUs

	Restricted Stock Units	Weighted- Average Grant Date Fair Value
Non-vested at December 31, 2012	192,308	$	8.80
Vested	16,025	$	7.80
Non-vested at March 31, 2013	176,283	$	8.80