The Company’s 2014 Equity Incentive Plan, or the 2014 Plan, is administered by the Company’s Board of Directors or a duly authorized committee thereof, referred to as the Plan administrator. The 2014 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of equity compensation, collectively referred to as Stock Awards. Additionally, the 2014 Plan provides for the grant of performance cash awards. Incentive stock options may be granted only to employees. All other awards may be granted to employees, including officers, non-employee directors, and consultants. No incentive stock options may be granted under the 2014 Plan after the tenth anniversary of the effective date of the 2014 Plan. Stock Awards granted under the 2014 Plan vest at the rate specified by the Plan administrator. Initial grants of Stock Awards made to employees and non-employee consultants generally vest as to 25% on the first anniversary of the date of grant and the balance ratably over the next 36 months and subsequent grants vest monthly over a period of four years from the grant date. Stock options initially granted to members of the Company’s Board of Directors vest over a period of three years in equal installments from the date of the grant, subject to the option holder’s continued service as a Director through such date. Subsequent grants to Directors that are made automatically at Annual Meetings of Stockholders vest fully on the first anniversary of the date of grant. The Plan administrator determines the term of Stock Awards granted under the 2014 Plan up to a maximum of ten years.

The aggregate number of shares of the Company’s common stock reserved for issuance under the 2014 Plan has automatically increased on January 1 of each year, beginning on January 1, 2015 and will continue to increase on January 1 of each year through and including January 1, 2024, by 3% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors. On January 1, 2019, the aggregate number of shares of common stock that may be issued pursuant to Stock Awards under the 2014 Plan automatically increased from 4,900,481 to 6,086,907. The maximum number of shares that may be issued pursuant to the exercise of incentive stock options under the 2014 Plan is 30,000,000 shares.

In June 2019, the Board of Directors, upon the recommendation of the Compensation Committee, amended the Company’s non-employee director compensation policy. Pursuant to the terms of the amended policy, each non-employee director was entitled to receive, at the time of the Company’s 2019 Annual Meeting of Stockholders, 6,000 restricted stock units. As a result, on June 4, 2019, the date of the Company’s 2019 Annual Meeting of Stockholders, an aggregate of 24,000 restricted stock units were granted to Directors under the 2014 Plan with a grant date fair value of $20.47 per share. The restricted stock units vest on the earlier of (i) June 4, 2020 and (ii) immediately prior to the Company’s next Annual Meeting of Stockholders following the grant date, subject to the recipient’s continued service through such date. As a result, the Company will recognize compensation expense associated with these restricted stock units ratably over the one-year vesting period following the grant date. For the three and six months ended June 30, 2019, $41 of stock compensation expense relating to the Board of Directors’ restricted stock units was recognized in the Statements of Comprehensive Loss, all of which related to G&A expense. None of the 24,000 restricted stock units vested or were settled in shares of the Company’s common stock as of June 30, 2019.

In March 2019, the Compensation Committee of the Company’s Board of Directors approved and granted a total of 215,000 restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $16.10 per share. Vesting of the restricted stock units was contingent on the achievement of certain performance targets, subject to the recipient’s continuous service through the vesting events. At the date of grant, the Company concluded that the probability of achievement of the performance targets could not be determined until they were achieved, and accordingly, the Company would recognize compensation expense associated with these awards when, and to the extent, the restricted stock units vested in accordance with achievement of the performance targets. In May 2019, performance targets relating to 74,166 restricted stock units had been achieved and thus such restricted stock units vested and the awards were settled in shares of common stock. As a result, $1,194 of stock compensation expense relating to the vesting of these restricted stock units was recognized in the Statements of Comprehensive Loss for the three and six months ended June 30, 2019, consisting of $590 relating to G&A expense and $604 relating to R&D expense.

Under the 2014 Plan, the Company granted 241,000 and 136,500 stock options during the three months ended June 30, 2019 and 2018, respectively, and 1,198,000 and 732,500 stock options during the six months ended June 30, 2019 and 2018, respectively. The fair values of stock options granted during the three and six months ended June 31, 2019 and 2018 were estimated as of the dates of grant using the Black-Scholes option pricing model with the following assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	1.95% - 2.36%	2.85% - 2.94%	1.95% - 2.62%	2.51% - 2.94%
Expected volatility	73.06% - 73.78%	85.7% - 92.8%	73.06%-75.19%	85.7% - 92.8%
Expected dividend yield	0%	0%	0%	0%
Expected life of employee options (in years)	6.25	6.25	6.25	6.25
Expected life of non-employee options (in years)	—	—	—	—

The weighted-average grant date fair value per share of options granted to employees, non-employee members of the Company’s Board of Directors for their Board service and non-employee consultants during the three months ended June 30, 2019 and 2018 was $13.07 and $10.49, respectively, and during the six months ended June 30, 2019 and 2018 was $11.28 and $10.51, respectively.

Prior to January 1, 2019, the Company used the Black-Scholes option valuation model to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50. On January 1, 2019, the Company used the Black-Scholes option valuation model to re-measure the fair value of all outstanding unvested options that had been granted to non-employee consultants in accordance with ASU 2018-07 (see Note 2, Other Accounting Pronouncements Recently Adopted). The range of assumptions used by the Company on January 1, 2019 and June 30, 2018 are as follows:

	January 1,	June 30,
	2019	2018
Risk-free interest rate	2.59% - 2.62%	1.92% - 2.82%
Expected volatility	58.9% - 84.6%	60.4% - 96.2%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.81 - 8.19	0.25 - 8.69

During the three and six months ended June 30, 2019 and 2018, the Company recognized compensation expense relating to stock options (excluding compensation expense related to i) the issuance of common stock relating to the consulting agreement for $99 in G&A expense for the three and six months ended June 30, 2019; ii) the vesting of executives’ restricted stock units for $604 in R&D expense and $590 in G&A expense for the three and six months ended June 30, 2019; and iii) compensation expense relating to the Board of Directors’ restricted stock units for $41 in G&A expense for the three and six months ended June 30, 2019), as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	1,326	$	852	$	2,408		1,500
General and administrative		1,355		1,217		2,507		2,440
Total stock option expense	$	2,681	$	2,069	$	4,915	$	3,940

A summary of stock option award activity related to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as of and for the six months ended June 30, 2019 is presented below:

	Number of Shares			Weighted Average Exercise Price
Outstanding, December 31, 2018		4,004,422		$	13.34
Granted		1,198,000			16.77
Exercised		(395,997	)		10.63
Forfeited		(100,703	)		16.22
Outstanding, June 30, 2019		4,705,722		$	14.39
Options exercisable, June 30, 2019		2,034,916

The Company does not expect to realize any tax benefits from its stock option activity or the recognition of stock-based compensation expense because the Company currently has net operating losses and has a full valuation allowance against its deferred tax assets. Accordingly, no amounts related to excess tax benefits have been reported in cash flows from operations for the six months ended June 30, 2019 and 2018.