8. Equity Plans and Stock-Based Compensation

Option activity under our stock-based compensation plans during the six months ended June 30, 2017 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, 2016		3,975		$	21.38
Options granted		279			6.08
Options exercised		-			-
Options cancelled:
Options forfeited (unvested)		(310	)		18.24
Options cancelled (vested)		(193	)		29.32
Balance at June 30, 2017		3,751			20.09		6.09	$	1,136
Vested and expected to vest at June 30, 2017		3,741			20.10		6.09	$	1,136
Exercisable at June 30, 2017		2,290			22.34		5.57	$	151

In June 2017, 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 1,600,000 shares.

Restricted stock unit activity under our stock-based compensation plans during the six months ended June 30, 2017 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, 2016		699		$	12.12
Granted		2,124			4.76
Vested		(171	)		18.62
Forfeited or expired		(93	)		12.25
Non-vested as of June 30, 2017		2,559		$	5.58

The aggregate intrinsic value of the restricted stock units outstanding as of June 30, 2017, based on our stock price on that date, was $24.7 million. Fair value of restricted stock units is determined at the date of grant using our closing stock price.

As of June 30, 2017, approximately 110,000 shares underlying stock options and restricted stock units awards with performance-based vesting criteria were outstanding. Vesting criteria for these performance-based awards were not probable as of June 30, 2017.

Under our stock-based compensation plans, option awards generally vest over a three or four-year period contingent upon continuous service, and expire seven to 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,
	2017			2016			2017			2016			2017			2016
Weighted-average fair value	$	3.90		$	10.25		$	4.12		$	9.79		$	2.37		$	8.18
Risk-free interest rate		1.8	%		1.5	%		1.9	%		1.4	%		0.9	%		0.6	%
Expected life (in years)		4.5			5.4			4.5			5.0			1.2			1.2
Volatility		0.9			0.7			0.9			0.7			1.0			0.6

We recognized stock-based compensation expense of $3.4 million and $3.1 million for the three months ended June 30, 2017 and 2016, respectively. We recognized stock-based compensation expense of $7.2 million and $6.4 million for the six months ended June 30, 2017 and 2016, respectively. Compensation expense is based on awards ultimately expected to vest and reflects estimated forfeitures. The components of stock-based compensation expense were (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2017		2016		2017		2016
Research and development	$	1,771	$	1,336	$	3,734	$	2,851
General and administrative		1,592		1,788		3,450		3,517
Total	$	3,363	$	3,124	$	7,184	$	6,368

As of June 30, 2017, the total unrecognized compensation cost related to non-vested equity awards including all awards with time-based vesting amounted to $25.3 million, which is expected to be recognized over the remaining weighted-average vesting period of 1.9 years. Additionally, as of June 30, 2017, the total unrecognized compensation cost related to equity awards with performance-based vesting criteria not deemed probable of vesting amounted to $0.6 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. 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, 2017, employees have acquired 90,156 shares of our common stock under the Purchase Plan and 137,885 shares of our common stock remained available for future purchases under the Purchase Plan.