Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2012

Black-Scholes Valuation Model and Weighted-Average Assumptions

The fair value of each option is estimated on the date of grant using the Black-Scholes option valuation model and the following weighted-average assumptions:


	Employee Stock Options												Employee Stock Purchase Plan
	Three Months Ended June 30,						Six Months Ended June 30,						Six Months Ended June 30,
	2012			2011			2012			2011			2012			2011
Weighted-average fair value per share	$	3.34		$	2.30		$	3.30		$	2.78		$	3.80		$	2.52
Risk-free interest rate		0.55	%		1.3	%		0.53	%		1.6	%		0.19	%		0.52	%
Expected life (in years)		4.0			4.0			4.0			4.0			1.0			1.2
Volatility		1.6			1.6			1.6			1.6			1.6			1.6
Expected dividends		—			—			—			—			—			—

Stock-Based Compensation Expense

We recognized the following amounts of stock-based compensation expense (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2012		2011		2012		2011
Research and development expense	$	938	$	531	$	1,741	$	1,043
General and administrative expense		1,131		636		2,650		1,604

Total	$	2,069	$	1,167	$	4,391	$	2,647

Activity under Stock-Based Compensation Plans

Activity under the stock-based compensation plans during the six months ended June 30, 2012 was as follows (in thousands except per share amounts):


	Options and Awards Available for Grant			Number of Options and Awards Outstanding			Weighted-Average Price Per Share
Balance at December 31, 2011		11,524			11,101		$	3.09
Options granted		(3,612	)		3,612		$	3.66
Awards granted		(1,815	)		1,815		$	4.23
Options exercised		—			(824	)	$	1.70
Awards vested		—			(40	)	$	4.22
Options cancelled:
Options forfeited (unvested)		310			(310	)	$	3.24
Options expired (vested)		4			(4	)	$	4.55
Awards cancelled (unvested)		—			(40	)	$	4.22

Balance at June 30, 2012		6,411			15,310		$	3.42

Summary of Outstanding Options Net of Expected Forfeitures (Vested and Expected to Vest) and Options Exercisable under Stock Option Plans

The following table summarizes outstanding options that are net of expected forfeitures (vested and expected to vest) and options exercisable under our stock option plans as of June 30, 2012 (in thousands, except per share amounts and years):


	Number of Shares		Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding options (vested and expected to vest)		12,347	$	3.33		7.4	$	15,897
Options exercisable		6,689	$	3.53		6.1	$	8,930