The Company adopted the 2021 Employment Inducement Incentive Award Plan (the "2021 Inducement Plan") in December 2021. The Company initially reserved 1,500,000 shares of the Company’s common stock for issuance pursuant to awards granted under the 2021 Inducement Plan. The terms of the 2021 Inducement Plan are substantially similar to the terms of the Company’s 2018 Incentive Award Plan with the exception that awards may only be made to an employee who has not previously been an employee or member of the board of directors of the Company if the award is in connection with commencement of employment. No awards were granted under the 2021 Inducement Plan during the year ended December 31, 2021. In 2022, the Company amended the 2021 Inducement Plan to increase the number of shares of the Company’s common stock available for future issuance under the 2021 Inducement Plan to 5,000,000 shares. As of December 31, 2022, 2,315,100 shares of common stock were available for future issuance under the 2021 Inducement Plan.

The Company adopted the 2018 Incentive Award Plan (the “2018 Plan”) in July 2018. Under the 2018 Plan, which expires in July 2028, the Company may grant equity-based awards to individuals who are employees, officers, directors or consultants of the Company. Options issued under the 2018 Plan will generally expire ten years from the date of grant and vest over a four-year period. As of December 31, 2022, an aggregate of 2,167,691 shares of common stock were available for issuance under the 2018 Plan.

The 2018 Plan contains a provision that allows annual increases in the number of shares available for issuance on the first day of each calendar year through January 1, 2028 in an amount equal to the lesser of: (i) 5% of the aggregate number of

shares of the Company’s common stock outstanding on December 31 of the immediately preceding calendar year, or (ii) such lesser amount determined by the Company. Under this evergreen provision, on January 1, 2023, an additional 2,693,859 shares became available for future issuance under the 2018 Plan.

The Company adopted the 2015 Stock Incentive Plan (the “2015 Plan”) in February 2015, which provided for the issuance of equity awards to the Company’s employees, members of its board of directors and consultants. In general, options issued under this plan vest over four years and expire after 10 years. Subsequent to the adoption of the 2018 Plan, no additional equity awards can be made under the 2015 Plan.

Certain awards issued under the 2015 Plan allowed for exercise prior to vesting. Shares issued under such early-exercise provisions are subject to repurchase by the Company until they become fully vested. As of December 31, 2022, there were no unvested shares issued under early-exercise provisions were subject to repurchase by the Company. The consolidated balance sheet reflects an unvested stock liability of $2,000 as of December 31, 2021.

The Company adopted the 2018 Employee Stock Purchase Plan (the “ESPP”) in July 2018. The ESPP permits participants to purchase common stock through payroll deductions of up to 20% of their eligible compensation. As of December 31, 2022, an aggregate of 1,200,455 shares of common stock were available for issuance under the ESPP.

The ESPP contains a provision that allows annual increases in the number of shares available for issuance on the first day of each calendar year through January 1, 2028 in an amount equal to the lesser of: (i) 1% of the aggregate number of shares of the Company’s common stock outstanding on December 31 of the immediately preceding calendar year, or (ii) such lesser amount determined by the Company. Under this evergreen provision, on January 1, 2023, an additional 538,771 shares became available for future issuance under the ESPP.

Activity under the Company’s stock option plans during the year ended December 31, 2022 was as follows:


				Weighted-		Weighted-		Aggregate
				Average		Average		Intrinsic
	Options			Exercise		Remaining		Value
	Outstanding			Price		Term		(000’s)
Balance at December 31, 2021		6,553,594		$	16.07
Granted		4,139,204		$	19.54
Exercised		(524,774	)	$	9.00
Forfeited and expired		(410,695	)	$	19.28
Balance at December 31, 2022		9,757,329		$	17.79		8.0	$	23,149
Vested and expected to vest at December 31, 2022		9,757,329		$	17.79		8.0	$	23,149
Exercisable at December 31, 2022		4,122,612		$	15.90		6.6	$	18,493

Aggregate intrinsic value is calculated as the difference between the closing price of the Company’s common stock at December 31, 2022 and the exercise price of stock options that had strike prices below the closing price.

The total intrinsic value of options exercised during 2022, 2021 and 2020 was $5.9 million, $5.1 million and $2.5 million, respectively.

The total fair value of options vested during 2022, 2021 and 2020 was $23.0 million, $16.3 million and $9.9 million, respectively.

The Company’s restricted stock unit activity during the year ended December 31, 2022 was as follows:

The Company estimate the fair value of all stock option grants and ESPP using the Black-Scholes option pricing model and recognize forfeitures as they occur.

The following table summarizes the weighted average assumptions used to estimate the fair value of stock options granted under the Company’s stock option plans and the shares purchasable under the ESPP during the periods presented:


	Year ended December 31,
Stock Option Awards	2022			2021			2020
Expected term	6.1 years			6.0 years			6.0 years
Expected volatility		88	%		86	%		78	%
Risk free interest rate		2.8	%		1.0	%		0.8	%
Expected dividend yield		—	%		—	%		—	%


	Year ended December 31,
ESPP	2022			2021			2020
Expected term	1.3 years			1.6 years			1.4 years
Expected volatility		87	%		91	%		84	%
Risk free interest rate		4.1	%		0.3	%		0.2	%
Expected dividend yield		—	%		—	%		—	%

The weighted-average fair value of stock options granted to employees during the years ended December 31, 2022, 2021 and 2020 was $14.50, $13.02 and $12.83 per share, respectively. The weighted-average fair value of awards under the ESPP during the years ended December 31, 2022, 2021 and 2020 was $9.39, $12.52 and $8.07 per share, respectively.

The key assumptions used in determining the fair value of equity awards, and the Company’s rationale, were as follows: (i) Expected term - the expected term for options represents the period that options are expected to be outstanding and has been estimated using the simplified method, which is an average of the contractual option term and its vesting period; the expected term for ESPP represents the term the awards are expected to be outstanding; (ii) Expected volatility - the expected volatility assumption is based on volatilities of a peer group of similar companies in the biotechnology industry whose share prices are publicly available. The Company computes the historical volatility data using the daily close prices for the selected companies’ shares during the equivalent period of the calculated expected term of the Company’s stock-based awards. The Company will continue to apply this process until sufficient historical information regarding the volatility of its common stock price becomes available or the Company believes the volatility of its own market-traded shares best represent expected volatility; (iii) Risk-free interest rate - the risk-free interest rate is based on the U.S. Treasury yield in effect at the time of grant for zero coupon U.S. Treasury notes with maturities that approximate the expected terms of awards; and (iv) Expected dividend yield - the expected dividend yield assumption is zero as the Company has never paid dividends and has no present intention to do so in the future.

Stock-based compensation expense for the equity awards issued by the Company to employees and non-employees for the periods presented below was as follows (in thousands):


	Year ended December 31,
	2022		2021		2020
Included in research and development	$	15,078	$	9,654	$	5,218
Included in general and administrative		13,190		7,698		5,209
Total stock-based compensation expense	$	28,268	$	17,352	$	10,427

As of December 31, 2022, unrecognized stock-based compensation cost related to option awards, restricted stock units, and ESPP was $72.5 million, $4.5 million and $2.8 million, respectively, which is expected to be recognized over a remaining weighted-average period of approximately 2.1 years, 3.1 years and 1.8 years, respectively.