Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2013

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation Expense

The following table shows total stock-based compensation expense recognized in the consolidated statements of operations (in thousands):

	Year Ended December 31,
	2013		2012		2011
Research and development	$	3,204	$	2,892	$	3,769
General and administrative		2,942		2,446		4,312
Total stock-based compensation expense	$	6,146	$	5,338	$	8,081

Assumptions Used for Estimating Fair Value of Employee Stock Options

The assumptions used for estimating the fair value of the employee stock options are as follows:

	Year Ended December 31,
	2013			2012			2011
Risk-free interest rate		1.50-1.88	%		0.74-1.34	%		0.93-2.11	%
Expected life of option (in years)		5.36-5.38			5.40-5.58			5.39-5.41
Expected dividend yield of stock		0	%		0	%		0	%
Expected volatility		0.88			0.87-0.88			0.83-0.86

Weighted-Average Assumptions Used for Estimating Fair Value of ESPP Purchase Rights

The weighted-average estimated fair value of shares purchased under the Company’s ESPP during 2013, 2012 and 2011 were $1.47, $2.51 and $1.62, respectively, based upon the assumptions in the Black-Scholes valuation model. The weighted–average assumptions used for estimating the fair value of the ESPP purchase rights are as follows:

	Year Ended December 31,
	2013			2012			2011
Risk-free interest rate		0.05-0.61	%		0.05-0.30	%		0.05-0.61	%
Expected life of option (in years)		0.5-2.0			0.5-2.0			0.5-2.0
Expected dividend yield of stock		0	%		0	%		0	%
Expected volatility		0.51-0.85			0.93-1.37			0.58-0.85