8. Equity Plans and Stock-Based Compensation

Option activity under our stock-based compensation plans during the six months ended June 30, 2016 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, 2015		2,891		$	23.34
Options granted		1,173			20.59
Options exercised		(14	)		13.73
Options cancelled:
Options forfeited (unvested)		(27	)		18.26
Options cancelled (vested)		(47	)		45.02
Balance at June 30, 2016		3,976			22.34		6.96	$	373
Vested and expected to vest at June 30, 2016		3,926			22.41		6.94	$	373
Exercisable at June 30, 2016		1,641			25.93		5.23	$	364

In May 2016, stockholders of the Company approved a proposal to increase the aggregate number of shares of common stock authorized for issuance under the 2011 Equity Incentive Plan, as amended, by 3,200,000 shares.

Restricted stock unit activity under our stock-based compensation plans during the six months ended June 30, 2016 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, 2015		195		$	17.52
Granted		379		$	21.48
Vested		(23	)	$	15.55
Forfeited or expired		(12	)	$	20.44
Non-vested as of June 30, 2016		539		$	20.33

The aggregate intrinsic value of the restricted stock units outstanding as of June 30, 2016, based on our stock price on that date, was $7.9 million.

As of June 30, 2016, approximately 189,000 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, 2016.

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,
	2016			2015			2016			2015			2016			2015
Weighted-average fair value	$	10.25		$	14.28		$	9.79		$	12.76		$	8.18		$	8.18
Risk-free interest rate		1.5	%		1.7	%		1.4	%		1.7	%		0.6	%		0.4	%
Expected life (in years)		5.4			5.9			5.0			5.9			1.2			1.2
Volatility		0.7			0.7			0.7			0.7			0.6			0.7

We recognized stock-based compensation expense of $3.1 million and $1.9 million for the three months ended June 30, 2016 and 2015, respectively. We recognized stock-based compensation expense of $6.4 million and $3.9 million for the six months ended June 30, 2016 and 2015, respectively. The components of stock-based compensation expense were (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2016		2015		2016		2015
Research and development	$	1,336	$	868	$	2,851	$	1,753
General and administrative		1,788		1,050		3,517		2,119
Total	$	3,124	$	1,918	$	6,368	$	3,872

As of June 30, 2016, the total unrecognized compensation cost related to non-vested equity awards including all awards with time-based vesting amounted to $21.8 million, which is expected to be recognized over the remaining weighted-average vesting period of 2.6 years. Additionally, as of June 30, 2016, the total unrecognized compensation cost related to equity awards with performance-based vesting criteria not deemed probable of vesting amounted to $6.8 million.

Employee Stock Purchase Plan

The 2014 Employee Stock Purchase Plan, as amended, (the “Purchase Plan”) provides for the purchase of common stock by eligible employees and became effective on May 28, 2014. In May 2016, stockholders of the Company approved a proposal to increase the aggregate number of shares of common stock authorized for issuance under the Purchase Plan by 200,000. 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, 2016, employees have acquired 40,670 shares of our common stock under the Purchase Plan and 209,330 shares of our common stock remained available for future purchases under the Purchase Plan.