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.

During the three months ended March 31, 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. Vesting of the restricted stock units will be contingent on the achievement of certain performance targets, subject to the recipient’s continuous service through the vesting events. The Company has determined that the probability of achievement of the performance targets cannot be determined until they are achieved, and accordingly, the Company intends to recognize compensation expense associated with these awards when, and to the extent, the restricted stock units vest in accordance with achievement of the performance targets. As of March 31, 2019, none of the restricted stock units had vested and, consequently, no compensation expense has been recognized.

Under the 2014 Plan, the Company granted 957,000 and 596,000 stock options during the three months ended March 31, 2019 and 2018, respectively. The fair values of stock options granted during the three months ended March 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 March 31,
	2019	2018
Risk-free interest rate	2.54% - 2.62%	2.51% - 2.71%
Expected volatility	74.29% - 75.19%	85.9%
Expected dividend yield	0%	0%
Expected life of employee options (in years)	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 March 31, 2019 and 2018 was $10.83 and $10.43, 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 March 31, 2018 are as follows:

	January 1,	March 31,
	2019	2018
Risk-free interest rate	2.59% - 2.62%	1.82% - 2.70%
Expected volatility	58.9% - 84.6%	78.2% - 101.0%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.81 - 8.19	0.50 - 8.94

During the three months ended March 31, 2019 and 2018, the Company recognized compensation expense in the accompanying Condensed Statements of Comprehensive Loss relating to stock options, as follows:

	Three Months Ended March 31,
	2019		2018
Research and development	$	1,082	$	649
General and administrative		1,152		1,222
Total stock option expense	$	2,234	$	1,871

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 three months ended March 31, 2019 is presented below:

	Number of Shares			Weighted Average Exercise Price
Outstanding, December 31, 2018		4,004,422		$	13.34
Granted		957,000			16.06
Exercised		(17,291	)		13.52
Forfeited		(18,490	)		16.55
Outstanding, March 31, 2019		4,925,641		$	13.86
Options exercisable, March 31, 2019		2,134,424

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 three months ended March 31, 2019 and 2018.