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. 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. In November 2023, 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 7,500,000 shares. As of June 30, 2024, 1,857,809 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 June 30, 2024, 2,868,497 shares of common stock were available for future 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, 2024, an additional 3,408,761 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.

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 June 30, 2024, 2,157,852 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, 2024, an additional 681,752 shares became available for future issuance under the ESPP.

Activity under the Company’s stock option plans during the six months ended June 30, 2024 was as follows:


				Weighted-		Weighted-		Aggregate
				Average		Average		Intrinsic
	Options			Exercise		Remaining		Value
	Outstanding			Price		Term		(000’s)
Balance at December 31, 2023		12,627,124		$	18.96
Granted		3,061,753		$	43.60
Exercised		(1,230,857	)	$	16.64
Forfeited and expired		(319,775	)	$	23.08
Balance at June 30, 2024		14,138,245		$	24.40		8.0	$	288,961
Vested and expected to vest at June 30, 2024		14,138,245		$	24.40		8.0	$	288,961
Exercisable at June 30, 2024		5,796,606		$	18.69		6.7	$	151,303

Aggregate intrinsic value is calculated as the difference at a specific point in time between the closing price of the Company’s common stock on June 28, 2024, the last trading day of the quarter, and the exercise price of stock options that had exercise prices below the closing price. The aggregate intrinsic value of options exercised during the six months ended June 30, 2024 and 2023 was $34.0 million and $0.8 million, respectively.

The Company’s restricted stock unit activity during the six months ended June 30, 2024, was as follows:


				Weighted-
	Restricted Stock			Average
	Units			Grant Date
	Outstanding			Fair Value
Balance at December 31, 2023		813,634		$	19.71
Granted		897,686		$	43.54
Vested		(243,910	)	$	19.85
Forfeited		(37,574	)	$	31.46
Balance at June 30, 2024		1,429,836		$	34.34

The Company estimates the fair value of all stock option grants and the ESPP using the Black-Scholes option pricing model and recognizes 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 for the periods presented below:


Stock Option Awards	Three months ended June 30,		Six months ended June 30,
	2024	2023	2024	2023
Expected option term	6.0 years	6.0 years	6.0 years	6.0 years
Expected volatility	66%	65%	66%	66%
Risk free interest rate	4.5%	3.6%	4.3%	4.0%
Expected dividend yield	—%	—%	—%	—%

The weighted-average fair value of stock options awarded was $29.32 and $27.73 per share during the three and six months ended June 30, 2024, respectively, and $12.56 and $12.38 per share during the three and six months ended June 30, 2023, respectively.

The following table summarizes the weighted average assumptions used to estimate the fair value of the ESPP for the periods presented below:


ESPP	Three and six months Ended June 30,
	2024	2023
Expected term	1.1 years	1.1 years
Expected volatility	74%	57%
Risk free interest rate	5.1%	4.9%
Expected dividend yield	—%	—%

The weighted-average fair value of the ESPP was $22.07 per share during each of the three and six months ended June 30, 2024, respectively, and $9.15 per share during each of the three and six months ended June 30, 2023, 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 stock options represents the period that the stock options are expected to be outstanding and has been estimated using the simplified method, due to limited historical exercise behavior. The expected term using the simplified method is an average of the contractual option term and its vesting period; the expected term for awards granted under the ESPP represents the term the awards are expected to be outstanding; (ii) Expected volatility - the expected volatility assumption is based on the historical volatility of the Company's common stock; (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.

Restricted stock units are valued using the closing price of our common stock on the date of grant.

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):


	Three months ended June 30,				Six months ended June 30,
	2024		2023		2024		2023
Included in research and development	$	11,491	$	5,602	$	19,056	$	10,280
Included in general and administrative		7,449		4,573		13,338		7,991
Total stock-based compensation expense	$	18,940	$	10,175	$	32,394	$	18,271

As of June 30, 2024, unrecognized stock-based compensation cost related to option awards, restricted stock units, and ESPP was $148.4 million, $44.7 million and $3.2 million, respectively, which is expected to be recognized over a remaining weighted-average period of 2.1 years, 3.2 years and 1.4 years, respectively.