Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2014

Stock-Based Compensation Expense Related to Common Stock Options, Common Stock, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, common stock, restricted common stock and common stock warrants:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2014		2013		2014		2013
	(in thousands)
Research and development	$	224	$	335	$	1,083	$	865
General and administrative		653		1,473		2,188		2,411
Total stock-based compensation expense	$	877	$	1,808	$	3,271	$	3,276

Summary of Stock Option Activity

The following table summarizes the stock option activity in the Company’s equity incentive plans, including non-plan grants to Company executives, from December 31, 2013 through September 30, 2014:

	Shares			Weighted Average Exercise Price
Outstanding, December 31, 2013		3,348,705		$	4.70
Granted		354,823			12.72
Exercised		(350,728	)		1.91
Options forfeited/cancelled		(24,283	)		3.85
Outstanding, September 30, 2014		3,328,517		$	4.82

Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of all other options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

	Nine Months Ended September 30,
	2014			2013
Risk-free interest rate		1.58	%		1.17	%
Expected life of the options		6.0 years			5.3 years
Expected volatility of the underlying stock		114	%		115	%
Expected dividend rate		0	%		0	%