Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2013

Stockholders' Equity

Summary of information related to stock compensation expense recognized in the statements of operations

The following table summarizes information related to stock compensation expense recognized in the statements of operations (in thousands):

	Three Months Ended March 31,
	2012		2013
Stock compensation expense recognized in:
Research and development expense	$	873	$	924
General and administrative expense	515		650
Total stock compensation expense	$	1,388	$	1,574

Schedule of fair value weighted-average assumptions

	Three Months Ended March 31,
	2012			2013
Expected stock price volatility	78.4		%	82.0		%
Risk free interest rate	1.1		%	1.2		%
Expected life of options (years)	6.25			6.25
Expected annual dividend per share	$	0.00		$	0.00

Summary of option activities

					Weighted
			Weighted		Average
	Number		Average		Remaining	Aggregate
	of		Exercise		Contractual	Intrinsic
	Shares		Price		Life	Value
	(in thousands)					(in thousands)
Balance at December 31, 2012	7,974.2		$	6.35
Options granted	1,154.5		$	3.53
Options exercised	—		$	—
Options forfeited	(47.6	)	$	4.94
Balance at March 31, 2013	9,081.1		$	5.99	7.51 years	$	207
Vested and unvested expected to vest, March 31, 2013	8,012.5		$	6.10	7.3 years	$	132
Exercisable at March 31, 2013	4,450.9		$	7.25	6.0 years	$	166