In December 2020, the Company adopted the 2020 Incentive Award Plan (“2020 Plan”), which became effective on December 10, 2020. The 2020 Plan initially reserved 2,606,546 shares of common stock for the issuance of stock options, stock appreciation rights, restricted stock awards, restricted stock units, performance bonus awards, performance stock units, dividend equivalents or other stock or cash based award granted to employees, directors and consultants of the Company. The number of shares reserved for future issuance under the 2020 Plan will increase annually on the first day of each fiscal year beginning in 2021 and ending in 2030 by the lesser of (i) 5% of the shares of common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company's board of directors, provided, however, no more than 18,000,000 shares of the Company's common stock may be issued upon the exercise of incentive stock options. In 2022, an additional 1,611,226 shares of the Company's common stock became available for issuance under the 2020 Plan, as a result of the operation of the automatic annual increase provision. All options are exercisable over a period not to exceed the contractual term of ten years from the date the stock options were issued. As of June 30, 2022 there were 2,902,219 shares available for grant under the 2020 Plan.

Following the effectiveness of the 2020 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 of options that remain outstanding under the 2015 Plan. All outstanding grants under the 2015 Plan that are repurchased, forfeited, expire or are returned back to the 2015 Plan are not available for grant under the 2020 Plan.

The 2015 Plan provided for grants of stock options, stock appreciation rights, restricted stock and restricted stock unit awards to employees, directors and consultants of the Company. As of June 30, 2022, options to purchase 2,371,439 shares of the Company's common stock were outstanding under the 2015 Plan. All options are exercisable over a period not to exceed the contractual term of ten years from the date the stock options were issued and are granted at prices not less than the estimated fair market value of the Company’s common stock on the grant date as determined by the Company's board of directors. If an

individual owns stock representing more than 10% of the Company’s outstanding shares, the exercise price of each share shall be at least 110% of the fair market value on the date of grant.

In December 2020, the Company adopted the 2020 Employee Stock Purchase Plan (the “2020 ESPP”). Under the 2020 ESPP, 252,337 shares of the Company's common stock were initially reserved for employee purchases of the Company's common stock under terms and provisions established by the Company's board of directors and approved by the Company's stockholders. The number of shares reserved for future issuance under the 2020 ESPP will increase annually on the first day of each fiscal year beginning in 2021 and ending in 2030 by the lesser of (i) 1% of the shares of the Company's common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company's board of directors, provided, however, no more than 15,000,000 shares of the Company's common stock may be issued under the 2020 ESPP. On March 28, 2022, an additional 25,000 shares of the Company's common stock became available for future issuance under the 2020 ESPP, as a result of the operation of this automatic annual increase provision.

Under the 2020 ESPP the Company's employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2020 ESPP provides for a series of overlapping 24-month offering periods comprised of four six-month purchase periods. The initial offering period under the ESPP is longer than 24 months, commencing February 15, 2021 and ending on May 14, 2023. Contributions under the 2020 ESPP are limited to a maximum of 15% of an employee's eligible compensation.

The following table summarizes the stock options activity for the six months ended June 30, 2022:


	Number of Shares Available for Grant			Number of Shares Underlying Outstanding Options
Balances at December 31, 2021		2,690,097			3,881,274
Options authorized		1,611,226			—
Options granted		(1,497,137	)		1,497,137
Options exercised		—			(90,802	)
Options expired		55,438			(55,438	)
Options forfeited		211,301			(211,301	)
Balances at June 30, 2022		3,070,925			5,020,870
Options exercisable at June 30, 2022					2,012,208
Options vested and expected to vest at June 30, 2022					5,020,870

The Company estimates the fair value of stock options using the Black-Scholes option pricing model. The fair value of employee and nonemployee options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company’s stock options was estimated using the following assumptions for the three and six months ended June 30, 2022 and 2021.


	Three Months Ended June 30,		Six Months Ended June 30,
	2022	2021	2022	2021
Expected term	5.9 - 6.1 years	5.8 - 6.1 years	5.9 - 6.1 years	5.8 - 6.3 years
Expected volatility	80.6% - 81.3%	82.8% - 84.3%	80.0% - 81.3%	82.8% - 84.3%
Risk-free interest rate	2.7% - 3.6%	1.0% - 1.1%	1.5% - 3.6%	0.6% - 1.1%
Expected dividend yield	—%	—%	—%	—%

Expected Term. The expected term for employee options is calculated using the simplified method as the Company does not have sufficient historical information to provide a basis for this estimate. The

simplified method is based on the average of the vesting tranches and the contractual life of each grant. The expected term for nonemployee options is the contractual term of the options.

Expected Volatility. The expected volatility is based on a mix of the Company's historical volatility and the historical volatility of comparable companies with similar attributes to the Company, including industry, stage of life cycle, size and financial leverage. For each grant, the Company measured historical volatility over a period equivalent to the expected term.

Risk-free Interest Rate. The risk-free interest rate is based on the yield available on U.S. Treasury zero-coupon issues whose term is similar in duration to the expected term of the respective stock option.

Expected Dividend Yield. The Company has not paid and does not anticipate paying any dividends on its common stock in the future. Accordingly, the Company has estimated the dividend yield to be zero.

The following table is a summary of stock-based compensation expense incurred for employees and nonemployees by function (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Research and development	$	2,343	$	1,895	$	4,493	$	3,008
General and administrative		1,979		1,921		3,762		3,137
Total stock-based compensation expense	$	4,322	$	3,816	$	8,255	$	6,145