In May 2019, the Company’s board of directors adopted and the Company’s stockholders approved the 2019 Incentive Award Plan (the “2019 Plan”), under which the Company may grant cash and equity-based incentive awards to the Company’s employees, consultants and directors. Following the effectiveness of the 2019 Plan, the Company will not make any further grants under the 2015 Equity Incentive Plan (the “2015 Plan”). However, the 2015 Plan continues to govern the terms and conditions of the outstanding awards granted under it. Shares of common stock subject to awards granted under the 2015 Plan that are forfeited or lapse unexercised and which following the effective date of the 2019 Plan are not issued under the 2015 Plan will be available for issuance under the 2019 Plan.

Options granted under the 2019 Plan may be either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). ISOs may be granted only to Company employees (including officers and directors who are also employees). NSOs may be granted to Company employees and consultants.

The exercise price of an ISO and NSO shall not be less than 100% of the estimated fair value of the shares on the date of grant. The exercise price of an ISO granted to an employee who, at the time of grant, owns stock representing more than 10% of the voting power of all classes of stock of the Company (a “10% stockholder”) shall be no less than 110% of the estimated fair value of the shares on the date of grant. Options granted under the 2019 Plan have a term of 10 years (or five years if granted to a 10% stockholder) and generally vest over a 4-year period with 1-year cliff vesting.

In 2015, the Company established its 2015 Plan which provides for the granting of stock options to employees and consultants of the Company. Options granted under the 2015 Plan may be either ISOs or NSOs.

In May 2019, the Company’s board of directors adopted and the Company’s stockholders approved the 2019 Employee Stock Purchase Plan (the “ESPP”). The ESPP provides eligible employees with the opportunity to acquire an ownership interest in the Company through periodic payroll deductions up to 15% of eligible compensation. The offering period is determined by the Company in its discretion but may not exceed 27 months. The per-share purchase price on the applicable exercise date for an offering period is equal to the lesser of 85% of the fair market value of the common stock at either the first business day or last business day of the offering period, provided that no more than 4,000 shares of common stock may be purchased by any one employee during each offering period. The ESPP is intended to constitute an “employee stock purchase plan” under Section 423(b) of the Internal Revenue Code of 1986, as amended. A total of 195,000 shares of common stock were initially reserved for issuance under the ESPP, subject to an annual increase on January 1 of each year, beginning on January 1, 2020. For the years ended December 31, 2020 and December 31, 2019, the Company recorded $0.2 million and less than $0.1 million of compensation expense related to participation in the ESPP.

Total stock-based compensation expense recorded related to awards granted to employees and non-employees was as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	1,234	$	956	$	410
General and administrative		2,375		1,212		540
Total stock-based compensation expense	$	3,609	$	2,168	$	950

				Outstanding Options
	Shares available for Grant			Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Balance, January 1, 2020		1,036,746			1,962,332		$	6.03		8.63
Additional shares authorized		813,578			—
Options granted		(1,252,826	)		1,252,826		$	8.48
Options exercised		—			(256,583	)	$	4.68
Options repurchased		10,518			—		$	1.08
Options canceled		367,119			(367,119	)	$	6.58
Balance, December 31, 2020		975,135			2,591,456		$	7.27		8.36
Exercisable as of December 31, 2020					844,226		$	5.79		7.55
Vested and expected to vest as of December 31, 2020					2,591,456		$	7.27		8.36

The weighted-average grant-date fair value of options granted during the years ended December 31, 2020 and December 31, 2019 was $6.16 and $6.73 per share, respectively. The aggregate intrinsic value of options exercised for the years ended December 31, 2020 and December 31, 2019 was $1.7 million and $0.8 million, respectively. Intrinsic values are calculated as the difference between the exercise price of the underlying options and the fair value of the common stock on the date of exercise.

As of December 31, 2020, the total unrecognized stock-based compensation expense for stock options was $8.5 million, which is expected to be recognized over a weighted-average period of 2.51 years.

The terms of the 2015 Plan permit the exercise of options granted under the 2015 Plan prior to vesting, subject to required approvals. The shares so acquired prior to vesting are subject to a lapsing repurchase right in favor of the Company at the original purchase price of such shares, exercisable upon a termination of the holder’s service with the Company prior to full vesting. The proceeds are initially recorded in other liabilities from the early exercise of stock options and are reclassified to additional paid-in capital as the Company’s repurchase right lapses. During the years ended December 31, 2020 and December 31, 2019, the Company repurchased 10,518 and 23,698 shares of common stock, respectively. As of December 31, 2020 and December 31, 2019, shares that were subject to repurchase were 14,460 and 84,964, respectively. The aggregate exercise price of early exercised shares as of December 31, 2020 and December 31, 2019 was less than $0.1 million and $0.1 million, respectively, which were recorded in other current liabilities and other non-current liabilities.

The fair values of options were calculated using the assumptions set forth below:

	Year Ended December 31,
	2020	2019	2018
Expected term	5.5 - 6.1 years	5.4 - 6.1 years	5.8 - 6.1 years
Expected volatility	84.9 % - 98.3%	77.2% - 85.5%	80.3% - 82.3%
Risk-free interest rate	0.3% - 1.5%	1.4% - 2.5 %	2.6% - 3.1%
Dividend yield	0%	0%	0%

Expected term. The expected term represents the weighted-average period the stock options are expected to remain outstanding and is based on the options’ vesting terms, contractual terms and industry peers, as the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior.

Expected Volatility. The Company uses an average historical stock price volatility of a peer group of publicly traded companies to be representative of its expected future stock price volatility, as the Company does not have sufficient trading history for its common stock. For purposes of identifying these peer companies, the Company considers the industry, stage of development, size and financial leverage of potential comparable companies. For each grant, the Company measures historical volatility over a period equivalent to the expected term. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available.

Risk-Free Interest Rate. The risk-free rate assumption is based on U.S. Treasury instruments whose term was consistent with the expected term of the Company’s stock options.

Expected Dividend Rate. The Company has not paid and does not anticipate paying any dividends in the near future. Accordingly, the Company has estimated the dividend yield to be zero.

The fair value of the Company’s common stock is determined based on its closing market price on the date of grant.

	Number of Shares Underlying Outstanding Restricted Stock Awards		Weighted Average Grant Date Fair Value
Unvested, December 31, 2019		14,625	$	0.82
Vested		—
Unvested, December 31, 2020		14,625	$	0.82

As of December 31, 2020 and December 31, 2019, 14,625 shares of restricted stock were outstanding with an aggregate purchase price of less than $0.1 million, which is recorded in other non-current liabilities on the balance sheets. The restricted stock vests upon the achievement of pre-defined research milestones. The holder of restricted stock has voting and dividend rights with respect to such shares held without regard to vesting. Shares of restricted stock are subject to a right of repurchase at the original purchase price held by the Company. As the restricted stock was purchased by an employee at a price equal to its fair value at the time of issuance, there was no stock-based compensation expense related to these awards. The total fair value of restricted stock vested during the years ended December 31, 2020 and December 31, 2019 was zero and $0.1 million in each period.