6. Stock Based Compensation

Our Board of Directors and the requisite stockholders previously approved the 2010 Equity Incentive Plan (the 2010 Plan). In October 2013, our Board of Directors approved the 2013 Equity Incentive Plan (the 2013 Plan) and in November 2013 our stockholders approved the 2013 Plan. The 2013 Plan became effective as of December 3, 2013, the date of the Company’s initial public offering (IPO). As of December 2, 2013, we suspended the 2010 Plan and no additional awards may be granted under the 2010 Plan. Any shares of common stock covered by awards granted under the 2010 Plan that terminate after December 2, 2013 by expiration, forfeiture, cancellation or other means without the issuance of such shares will be added to the 2013 Plan reserve.

As of June 30, 2017, the total number of shares of common stock available for issuance under the 2013 Plan is 8,637,790, which includes 2,684,456 of common stock that were available for issuance under the 2010 Plan as of the effective date of the 2013 Plan. Unless otherwise determined by the Board, beginning January 1, 2014, and continuing until the expiration of the 2013 Plan, the total number of shares of common stock available for issuance under the 2013 Plan will automatically increase annually on January 1 of each year by 4% of the total number of issued and outstanding shares of common stock as of December 31 of the immediate preceding year. Pursuant to approval by our board on January 1, 2017, the total number of shares of common stock available for issuance under the 2013 Plan was increased by 1,862,719 shares. As of June 30, 2017, a total of 4,790,350 options had been issued under the 2013 Plan.

In November 2013, our Board of Directors and stockholders approved the 2013 Employee Stock Purchase Plan (ESPP), which became effective as of December 5, 2013. We have reserved a total of 581,286 shares of common stock for issuance under the ESPP. Unless otherwise determined by our Board, beginning on January 1, 2014, and continuing until the expiration of the ESPP, the total number of shares of common stock available for issuance under the ESPP will automatically increase annually on January 1 by the lesser of (i) 1% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year, or (ii) 621,814 shares of common stock. Pursuant to approval by our board, there was no increase in the number of authorized shares in the ESPP in 2017. As of June 30, 2017, we have issued a total of 255,992 shares of common stock under the ESPP.

Total employee, director and non-employee stock-based compensation expense recognized for the three and six months ended June 30, 2017 are as follows (in thousands):


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2017		2016		2017		2016
General and administrative	$	1,416	$	874	$	2,884	$	1,826
Research and development		2,032		1,159		3,723		2,167
	$	3,448	$	2,033	$	6,607	$	3,993

The following table summarizes option activity under our stock plans and related information:


		Weighted		Weighted
		Average		Average
	Number of	Exercise		Remaining		Aggregate
	Shares subject	Price		Contractual		Intrinsic
	to outstanding	(Per		Term		Value
	options	Share)		(in years)		(in thousands)
Balances at December 31, 2016	4,045,801	$	11.95		7.82
Options granted	1,356,100	$	22.65
Options forfeited	(76,606)	$	15.60
Options exercised	(252,278)	$	6.66
Balance at June 30, 2017	5,073,017	$	15.02		7.92	$	33,950
Exercisable	2,206,587	$	10.22		6.60	$	24,051

We calculate the intrinsic value as the difference between the exercise price of the options and the closing price of common stock of $21.11 per share as of June 30, 2017.

Weighted average fair value of options granted during the three-month period ended June 30, 2017 and 2016 was $16.95 and $8.45 per share, respectively. There were 1,138,000 options granted during the six-month period ended June 30, 2016. We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following weighted average assumptions for the three and six months ended June 30, 2017 and 2016:


	Options				Options
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2017		2016		2017		2016
Expected term (years)	6.1		5.9		6.1		6.1
Expected volatility	88.9	%	76.1	%	89.2	%	75.8	%
Risk-free interest rate	1.91	%	1.36	%	2.04	%	1.53	%
Expected dividend yield	—	%	—	%	—	%	—	%


	ESPP				ESPP
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2017		2016		2017		2016
Expected term (years)	0.5 - 2.0		0.5 - 2.0		0.5 - 2.0		0.5 - 2.0
Expected volatility	67.8 - 79.8	%	67.8 - 79.6	%	67.8 - 79.8	%	67.8 - 79.6	%
Risk-free interest rate	.47% - 1.09	%	.47% - .93	%	.47% - 1.09	%	.47% - .93	%
Expected dividend yield	—	%	—	%	—	%	—	%

As of June 30, 2017 and 2016, the unamortized compensation expense related to unvested stock options was $35.4 million and $16.3 million, respectively. The remaining unamortized compensation expense will be recognized over the next three years. As of June 30, 2017 and 2016, the unamortized compensation expense under our ESPP was $242,305 and $491,000, respectively. The remaining unamortized expense will be recognized over the next 5.5 months.