9. Equity Plans and Stock-Based Compensation

Option activity under our stock-based compensation plans during the six months ended June 30, 2015 was as follows (in thousands except per share amounts):

	Shares Underlying Outstanding Options (in thousands)			Weighted-Average Exercise Price Per Share		Weighted-Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2014		1,820		$	27.48
Options granted		1,094			16.83
Options exercised		(8	)		15.60
Options cancelled:
Options forfeited (unvested)		(82	)		17.34
Options cancelled (vested)		(101	)		43.55
Balance at June 30, 2015		2,723			22.92		7.23	$	12,303
Vested and expected to vest at June 30, 2015		2,568			23.27		7.10	$	11,323
Exercisable at June 30, 2015		1,166			29.61		4.64	$	3,001

Restricted stock unit activity under our stock-based compensation plans during the six months ended June 30, 2015 was as follows (in thousands except per share amounts):

	Number of Shares (In thousands)			Weighted-Average Grant-Date Fair Value
Non-vested as of December 31, 2014		179		$	17.13
Granted		24		$	16.16
Vested		(3	)	$	16.90
Forfeited or expired		(5	)	$	18.71
Non-vested as of June 30, 2015		195		$	16.97

The aggregate intrinsic value of the restricted stock units outstanding as of June 30, 2015, based on our stock price on that date, was $4.6 million.

As of June 30, 2015, approximately 75,000 shares underlying stock options and restricted stock units awards with performance-based vesting criteria were outstanding. Vesting criteria for these performance-based awards have not been met as of June 30, 2015.

Under our stock-based compensation plans, option awards generally vest over a four-year period contingent upon continuous service and expire ten years from the date of grant (or earlier upon termination of continuous service). The fair value-based measurement of each option is estimated on the date of grant using the Black-Scholes option valuation model.

The fair value-based measurements and weighted-average assumptions used in the calculations of these measurements are as follows:

	Stock Options						Stock Options						Employee Stock Purchase Plan
	Three Months Ended						Six Months Ended						Six Months Ended
	June 30,						June 30,						June 30,
	2015			2014			2015			2014			2015			2014
Weighted-average fair value	$	14.28		$	13.60		$	12.76		$	15.50		$	8.18		$	9.20
Risk-free interest rate		1.7	%		1.9	%		1.7	%		1.8	%		0.4	%		0.2	%
Expected life (in years)		5.9			5.8			5.9			5.9			1.2			1.1
Volatility		0.7			1.3			0.7			1.4			0.7			0.9

We recognized stock-based compensation expense of $1.9 million and $1.7 million for the three months ended June 30, 2015 and 2014, respectively. We recognized stock-based compensation expense of $3.9 million and $2.9 million for the six months ended June 30, 2015 and 2014, respectively. The components of stock-based compensation expense were (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2015		2014		2015		2014
Research and development	$	868	$	738	$	1,753	$	1,430
General and administrative		1,050		938		2,119		1,516
Total	$	1,918	$	1,676	$	3,872	$	2,946

As of June 30, 2015, the total unrecognized compensation cost related to non-vested equity awards including all awards with time-based vesting amounted to $21.4 million, which is expected to be recognized over the remaining weighted-average vesting period of 3.0 years. Additionally, as of June 30, 2015, the total unrecognized compensation cost related to equity awards with performance-based vesting criteria not deemed probable of vesting amounted to $0.4 million.

Employee Stock Purchase Plan

As of June 30, 2015, 99,600 shares were approved for issuance under the 2004 Employee Stock Purchase Plan, subject to adjustment for a stock split, or any future stock dividend or other similar change in our common stock or capital structure. As of June 30, 2015, employees have acquired 94,410 shares of our common stock under the 2004 Employee Stock Purchase Plan. As of June 30, 2015, 5,190 shares of our common stock remained available for future purchases under the 2004 Employee Stock Purchase Plan.

In May 2014, stockholders of the Company approved the 2014 Employee Stock Purchase Plan (the “Purchase Plan”), pursuant to which the Company may issue up to 50,000 shares of its common stock, subject to adjustment, to its employees. The Purchase Plan provides for the purchase of common stock by eligible employees and became effective on May 28, 2014. The purchase price per share is the lesser of (i) 85% of the fair market value of the common stock on the commencement of the offer period (generally, the sixteenth day in February or August) or (ii) 85% of the fair market value of the common stock on the exercise date, which is the last day of a purchase period (generally, the fifteenth day in February or August). As of June 30, 2015, employees have acquired 7,959 shares of our common stock under the Purchase Plan and 42,041 shares of our common stock remained available for future purchases under the Purchase Plan.