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. On February 29, 2024, an additional 2,153,533 shares of 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 September 30, 2024 there were 1,210,988 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.

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 September 30, 2024, options to purchase 1,518,923 shares of 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 board of directors.

No additional grants will be made under the 2015 Plan, and all outstanding grants under the 2015 Plan that are repurchased, forfeited, expire or are cancelled are returned to the 2015 Plan and are not available for grant under the 2020 Plan.

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 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 the Company’s common stock may be issued under the 2020 ESPP. On February 29, 2024, an additional 50,000 shares of 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 comprising four six-month purchase periods. The initial offering period under the 2020 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 Company has granted restricted stock unit (RSU) awards under the 2020 Equity Incentive Award Plan that vest over a period of four years. The following table summarizes the RSU activity for the nine months ended September 30, 2024:


	Number of Shares
Unvested balance at December 31, 2023		181,181
Issued		214,025
Vested		(32,548	)
Canceled/Forfeited		(33,991	)
Unvested balance at September 30, 2024		328,667

The following table summarizes the stock options activity for the nine months ended September 30, 2024:


	Number of Shares Underlying Outstanding Options
Balances at December 31, 2023		8,240,154
Options granted		1,179,750
Options exercised		(367,962	)
Options expired		(11,395	)
Options forfeited		(215,224	)
Balances at September 30, 2024		8,825,323
Options exercisable at September 30, 2024		4,136,974
Options vested and expected to vest at September 30, 2024		8,825,323

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 nine months ended September 30, 2024 and 2023:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2024	2023	2024	2023
Expected term	6.0 - 6.1 years	6.0 - 6.1 years	6.0 - 6.1 years	6.0 - 6.1 years
Expected volatility	85.3% - 87.3%	80.6% - 82.2%	85.3% - 87.3%	80.6% - 84.4%
Risk-free interest rate	4.4% - 4.5%	4.1% - 4.4%	4.3% - 4.5%	3.5% - 4.4%
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 September 30,				Nine Months Ended September 30,
	2024		2023		2024		2023
Research and development	$	3,650	$	2,314	$	9,881	$	7,463
General and administrative		2,840		2,248		8,657		6,289
Total stock-based compensation expense	$	6,490	$	4,562	$	18,538	$	13,752