Stock-Based Compensation (Tables)

6 Months Ended

12 Months Ended

Jun. 30, 2013

Dec. 31, 2012

Summary of Option Activities

The following table summarizes option activity under the 2003 Plan and the 2013 Plan and related information during the six months ended June 30, 2013:

	Shares Available for Grant			Shares Subject to Outstanding Options			Weighted-Average Exercise Price  Per Share
Balance at December 31, 2012		593,011			3,451,178		$	6.35
Options authorized		150,000			—			—
Options granted		(560,854	)		560,854			13.98
Options exercised		—			(72,204	)		3.52
Options cancelled		151,913			(151,913	)		7.33
Balance at June 30, 2013		334,070			3,787,915		$	7.50

The following summarizes option activity under the 2003 Plan and related information during the years ended December 31, 2010, 2011 and 2012:

	Shares Available for Grant			Shares Subject to Outstanding Options			Weighted- Average Exercise Price Per Share
Balance at December 31, 2009		108,911			1,986,475		$	4.16
Options authorized		900,000			–			–
Options granted		(471,915	)		471,915			9.00
Options exercised		–			(52,127	)		3.36
Options canceled		12,670			(12,670	)		5.30
Options repurchased		167			–			–
Balance at December 31, 2010		549,833			2,393,593			5.13
Options authorized		250,000			–			–
Options granted		(850,977	)		850,977			8.35
Options exercised		–			(53,336	)		3.10
Options canceled		86,835			(86,835	)		5.86
Options repurchased		9,568			–			–
Balance at December 31, 2011		45,259			3,104,399			6.03
Options authorized		998,948			–			–
Options granted		(658,789	)		658,789			7.38
Options exercised		–			(104,417	)		3.04
Options canceled		207,593			(207,593	)		6.36
Balance at December 31, 2012		593,011			3,451,178		$	6.35

Additional Information Related to the Status of Options

Additional information related to the status of options at December 31, 2012, is as follows (aggregate intrinsic value in thousands):

	Shares		Weighted- Average Exercise Price Per Share		Remaining Contractual Life		Aggregate Intrinsic Value
Outstanding and exercisable		3,451,178	$	6.35		6.2	$	11,891
Vested and expected to vest		3,387,447	$	6.33		6.2	$	11,762
Vested		2,229,151	$	5.51		4.9	$	9,568

Stock Options Outstanding and Vested and Exercisable

Additional information regarding our stock options outstanding and vested and exercisable as of December 31, 2012 is summarized below:

	Options Outstanding and Exercisable						Options Vested
Exercise Prices	Number of Options Outstanding and Exercisable		Weighted Average Remaining Contractual Life (Years)		Weighted Average Exercise Price per Share		Number of Options Vested		Weighted Average Exercise Price Per Share
$1.00 - $2.00		56,914		1.8	$	1.27		56,914	$	1.27
$3.30		426,894		3.2		3.30		426,895		3.30
$3.60		10,000		4.2		3.60		10,000		3.60
$4.10		383,446		4.6		4.10		383,446		4.10
$4.50		2,500		4.5		4.50		2,500		4.50
$5.00		349,925		4.1		5.00		349,925		5.00
$5.10 - $5.30		364,471		4.8		5.22		355,155		5.22
$7.00		615,389		9.0		7.00		37,709		7.00
$8.50		708,349		8.0		8.50		300,740		8.50
$9.00 - $9.50		533,290		7.3		9.09		305,867		9.00
		3,451,178		6.2	$	6.35		2,229,151	$	5.51

Classification of Stock-Based Compensation Expense

The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands):

	Six Months Ended June 30,
	2012		2013
Research and development	$	749	$	945
General and administrative		662		978
Total stock-based compensation	$	1,411	$	1,923

Stock-based compensation expense, net of estimated forfeitures, is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2010		2011		2012
Research and development	$	1,170	$	1,164	$	1,452
General and administrative		764		1,189		1,357
Total stock-based compensation	$	1,934	$	2,353	$	2,809

Fair Values of Employee Stock Options

The estimated grant date fair values of the employee stock options were calculated using the Black-Scholes valuation model, based on the following assumptions:

	Six Months Ended June 30,
	2012	2013
Risk-free interest rate	1.2%	1.1%
Expected life	6.0 years	6.0 years
Volatility	70%	79%
Dividend yield	—	—

The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of options granted to employees:

	Year Ended December 31,
	2010	2011	2012
Risk-free interest rate	2.3%	1.1%	1.1%
Expected life	5.7 years	6.0 years	6.0 years
Volatility	78%	85%	72%
Dividend yield	–	–	–