Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2012

Stock-Based Compensation [Abstract]

Stock-based compensation expenses

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

	Year Ended December 31,
	2012		2011		2010
Research and development	$	2,892	$	3,769	$	3,612
General and administrative		2,446		4,312		4,206
Total stock-based compensation expense	$	5,338	$	8,081	$	7,818

Estimating the fair value of the employee stock options

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

	Year Ended December 31,
	2012			2011			2010
Risk-free interest rate		0.74-1.34	%		0.93-2.11	%		1.5-2.6	%
Expected life of option (in years)		5.40-5.58			5.39-5.41			5.23-5.41
Expected dividend yield of stock		0	%		0	%		0	%
Expected volatility		0.87-0.88			0.83-0.86			0.83-0.84

Summary of fair value of the employee stock options and ESPP plan

The weighted–average assumptions used for estimating the fair value of the ESPP purchase rights are as follows:

	Year Ended December 31,
	2012			2011			2010
Risk-free interest rate		0.05-0.30	%		0.05-0.61	%		0.2-1.0	%
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		0.93-1.37			0.58-0.85			0.62-1.14