8. Equity Plans and Stock-Based Compensation

Option activity under our stock-based compensation plans during the nine months ended September 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		329		$	6.99
Options exercised		(50	)	$	11.82
Options cancelled:
Options forfeited (unvested)		(351	)	$	18.43
Options cancelled (vested)		(217	)	$	29.11
Balance at September 30, 2017		3,686		$	20.05		5.79	$	13,420
Vested and expected to vest at September 30, 2017		3,671		$	20.06		5.79	$	13,390
Exercisable at September 30, 2017		2,394		$	22.18		5.40	$	5,948

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 nine months ended September 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		2,136			4.83
Vested		(171	)		18.62
Forfeited or expired		(221	)		8.44
Non-vested as of September 30, 2017		2,443		$	5.62

The aggregate intrinsic value of the restricted stock units outstanding as of September 30, 2017, based on our stock price on that date, was $52.5 million. Fair value of restricted stock units is determined at the date of grant using our closing stock price.

As of September 30, 2017, approximately 21,000 shares underlying stock options and approximately 63,000 restricted stock unit awards with performance-based vesting criteria were outstanding. Vesting criteria for these restricted stock units awards with performance-based awards were not probable as of September 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						Nine Months Ended						Nine Months Ended
	September 30,						September 30,						September 30,
	2017			2016			2017			2016			2017			2016
Weighted-average fair value	$	8.11		$	8.90		$	4.73		$	9.67		$	3.05		$	7.86
Risk-free interest rate		1.9	%		1.1	%		1.9	%		1.4	%		1.0	%		0.6	%
Expected life (in years)		4.5			4.5			4.5			4.9			1.2			1.2
Volatility		0.9			0.7			0.9			0.7			1.0			0.6

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				Nine Months Ended
	September 30,				September 30,
	2017		2016		2017		2016
Research and development	$	1,973	$	1,639	$	5,707	$	4,490
General and administrative		1,687		2,022		5,137		5,540
Total	$	3,660	$	3,661	$	10,844	$	10,030

As of September 30, 2017, the total unrecognized compensation cost related to non-vested equity awards including all awards with time-based vesting amounted to $22.0 million, which is expected to be recognized over the remaining weighted-average vesting period of 1.7 years. Additionally, as of September 30, 2017, 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

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). For the nine months ended September 30, 2017, employees have acquired 84,247 shares of our common stock under the Purchase Plan and 98,227 shares of our common stock remained available for future purchases under the Purchase Plan.