The Company’s 2014 Stock Incentive Plan (the “2014 Plan”) provides for the Company to grant incentive stock options or nonqualified stock options, restricted stock awards, unrestricted stock awards or restricted stock units to employees, directors and consultants of the Company. The 2014 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of the stock options may not be less than 100% of the fair market value of a share of the Company’s common stock on the date of grant and the term of the stock options may not be greater than ten years.

As of December 31, 2019 and 2018 all remaining shares available under the 2014 Plan were transferred to the 2018 Plan.

On April 3, 2018, the Company’s stockholders approved the 2018 Stock Option and Incentive Plan (the “2018 Plan”), which became effective on April 18, 2018, the date on which the registration statement for the Company’s initial public offering was declared effective. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, non-employee directors and other key persons (including consultants). The number of shares initially reserved for issuance under the 2018 Plan was 1,545,454, plus the shares of common stock remaining available for issuance under the 2014 Plan, which shall be cumulatively increased on each January 1 by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) under the 2018 Plan and the 2014 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan.

As of December 31, 2019 and 2018, 1,409,019 shares and 1,412,159 shares were available for future issuance under the 2018 Plan, respectively.

Stock options granted under the 2014 Plan and 2018 Plan to employees generally vest over four years and expire after ten years. The Company does not currently hold any treasury shares. Upon stock option exercise, the Company issues new shares and delivers them to the participant.

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2019			2018			2017
Risk-free interest rate		2.46	%		2.67	%		2.04	%
Expected term (in years)		6.12			6.18			6.25
Expected volatility		73.62	%		72.70	%		78.60	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The following table summarizes the Company’s stock option activity for the year ended December 31, 2019:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2018		4,414,225		$	6.79		8.29	$	2,031
Granted		1,518,500			4.06
Exercised		(110,156	)		2.31
Forfeited		(404,456	)		6.85
Outstanding as of December 31, 2019		5,418,113		$	6.11		7.69	$	690
Options exercisable at December 31, 2019		2,919,360		$	5.54		7.02	$	665
Vested and expected to vest at December 31, 2019		5,418,113		$	6.11		7.69	$	690

The weighted average grant-date fair value per share of stock options granted during the years ended December 31, 2019 and 2018, was $2.71 and $7.47, respectively.

The aggregate fair value of stock options vested during the years ended December 31, 2019 and 2018, was $6,634 and $4,493, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2019, 2018, and 2017 was $208, $1,887, and $1,876 respectively.

As of December 31, 2019, and 2018, there were outstanding stock options held by non-employees for the purchase of 272,343 and 317,957 shares of common stock, respectively, with service-based vesting conditions.

The Company has granted restricted common stock with service-based vesting conditions. The purchase price of the restricted common stock is determined by the Company’s board of directors. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the service-based vesting conditions of each award. The Company has the option to repurchase the restricted stock at the original purchase price if the grantee terminates its working relationship with the Company prior to the stock becoming vested.

As of December 31, 2019 and 2018 there was no unvested restricted common stock. The aggregate intrinsic value of restricted stock awards that vested during the year ended December 31, 2018 was $810.

On April 3, 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective on April 18, 2018, the date on which the registration statement for the Company’s initial public offering was declared effective. A total of 256,818 shares of common stock were initially reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP automatically increased on January 1, 2019, and shall increase each January 1 thereafter through January 1, 2028, by the lesser of (i) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 and (ii) such lesser number of shares as determined by the administrator of the Company’s ESPP. As of December 31, 2019, a total of 534,544 shares of common stock were reserved for issuance under this plan.

For the years ended December 31, 2019 and 2018, the Company did not issue any shares of common stock under the 2018 ESPP.

The Company recorded stock-based compensation expense related to stock options, restricted stock awards, and the ESPP in the following expense categories of its statements of operations and comprehensive loss:

	Year Ended December 31,
	2019		2018		2017
Research and development expenses	$	2,350	$	2,557	$	1,917
General and administrative expenses		3,641		2,660		2,792
	$	5,991	$	5,217	$	4,709

As of December 31, 2019, the Company had an aggregate of $10,733 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.25 years.