6. Stock Based Compensation

Our Board of Directors and the requisite stockholders previously approved the Amended and Restated 2000 Stock Incentive Plan (the 2000 Plan), and the 2010 Equity Incentive Plan (the 2010 Plan, and collectively with the 2000 Plan the Prior Plans). The 2000 Plan terminated August 2010 and no further awards may be issued under the 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 Prior Plans 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 September 30, 2015 the total number of shares of common stock available for issuance under the 2013 Plan is 6,335,940, which includes 2,684,456 of common stock that were available for issuance under the Prior Plans 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. As of September 30, 2015 a total of 1,994,750 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. As of September 30, 2015, a total of 581,286 shares of common stock have been reserved 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. As of September 30, 2015, we have issued a total of 111,852 shares of common stock under the ESPP.

Total employee, director and non-employee stock-based compensation expense recognized for the three months and nine months ended September 30, 2015 and 2014 are as follows (in thousands):


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014
General and administrative	$	593	$	276	$	1,628	$	575
Research and development		493		215		1,758		535
	$	1,086	$	491	$	3,386	$	1,110

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, 2014	2,826,794	$	5.12
Options granted	930,250	$	15.96
Options forfeited	(9,375)	$	18.75
Options exercised	(369,743)	$	1.47
Balance at September 30, 2015	3,377,926	$	8.47		7.21	$	16,173
Exercisable	1,515,353	$	3.69		5.15	$	12,965

We calculate the intrinsic value as the difference between the exercise price of the options and the closing price of common stock of $12.23 per share as of September 30, 2015.

Weighted average fair value of options granted during the nine-month period ended September 30, 2015 and 2014 was $10.73 and $7.11 per share, respectively. There were 1,019,021 options granted during the period ended September 30, 2014. 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 months and nine months ended September 30, 2015 and 2014:


	Options				Options
	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014
Expected term (years)	6.4		6.0		6.1		6.0
Expected volatility	86.5	%	77.4	%	76.6	%	77.4	%
Risk-free interest rate	1.95	%	2.02	%	1.62	%	2.02	%
Expected dividend yield	—	%	—	%	—	%	—	%


	ESPP				ESPP
	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014
Expected term (years)	0.5 - 2.0		0.5 - 2.0		0.5 - 2.0		0.5 - 2.0
Expected volatility	70.6 - 82.9	%	70.6	%	70.6 - 82.9	%	70.6	%
Risk-free interest rate	.06% - .46	%	.06% - .46	%	.06% - .46	%	.06% - .46	%
Expected dividend yield	—	%	—	%	—	%	—	%

At September 30, 2015, the Company had $11.3 million of total unrecognized compensation expense, net of estimated forfeitures, related to outstanding stock options and shares issued under our ESPP that will be recognized over the next 3.0 years.