The Company’s Board of Directors adopted, and its stockholders approved, an equity incentive plan in 2010 (as amended, the “2010 Plan”). The Board of Directors and stockholders amended the 2010 Plan in August 2017 to increase the number of shares of common stock reserved for issuance thereunder to 6,188,466. The 2010 Plan allowed for the grant of incentive stock options and non-qualified stock options to purchase common stock for employees, directors and consultants under terms and conditions established by the Board of Directors. Incentive stock options and nonqualified stock options were granted at exercise prices that were no less than 100% of the estimated fair value per share of the common stock on the date of grant. If an individual owns capital stock representing more than 10% of the voting shares, the price of each share was at least 110% of the fair value on the date of grant. The Board of Directors determined the fair value of common stock with the assistance of a third-party specialist. Options expire 10 years from the issuance date. Following the adoption of the 2017 Stock Incentive Plan, the Company no longer grants stock options or other awards under the 2010 Plan.

In October 2017, the Company’s Board of Directors adopted, and its stockholders approved, the 2017 Stock Incentive Plan (the “2017 Plan”), which became effective on November 8, 2017. The 2017 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, awards of restricted stock, restricted stock units and other stock-based awards. The number of shares of common stock reserved for issuance under the 2017 plan is the sum of (i) 1,359,587 shares of common stock, plus (ii) an additional number of shares of common stock equal to the sum of (a) the number of shares of common stock reserved for issuance under the 2010 equity incentive plan that remained available for future issuance immediately prior to the effectiveness of the 2017 Plan, which was 299,568 shares, and (b) the number of shares of common stock subject to outstanding awards under the 2010 equity incentive plan upon effectiveness of the 2017 plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right plus (iii) an annual increase, to be added the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors. In January 2024, the shares available for future issuance under the 2017 plan were increased by 4,219,409 shares pursuant to the annual increase described above. As of December 31, 2023, there were 6,497,120 shares available for future grants under the 2017 Plan.

Additionally, since 2019, the Company has granted equity awards as equity inducement awards material to entry into employment with the Company to certain newly hired employees outside of the Company’s existing plans in accordance with Nasdaq listing rule 5635(c)(4). In February 2020, the Board of Directors adopted the 2020 Inducement Stock Incentive Plan (the “2020 Plan”), which permitted the Company to grant equity awards to newly hired employees in accordance with Nasdaq listing rule 5635(c)(4). The aggregate number of shares reserved for issuance under the 2020 Plan was initially 750,000 shares. The Board of Directors amended the 2020 Plan to add 200,000 shares on January 1, 2024, which increased the total number of shares reserved for issuance to 1,950,000 shares as of January 1, 2024. As of December 31, 2023, there were 354,466 shares available for future grants under the 2020 Plan.

In October 2017, the Company’s board of directors adopted, and the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (“ESPP”), which became effective upon the IPO and provides participating employees with the opportunity to purchase up to an aggregate of 468,823 shares of common stock. The number of shares of common stock reserved for issuance under the 2017 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of (i) 937,646 shares of common stock, (ii) 1.0% of the number of shares of common stock outstanding on the first day of the fiscal year and (iii) an amount determined by the board of directors. The board of directors initiated the first offering under ESPP in October 2019.

The Company has reserved the following shares of common stock for future issuance (in thousands):


	December 31,
	2023		2022		2021
Shares reserved under 2017 Equity Incentive Plan		16,989		14,271		11,014
Shares reserved under 2017 Employee Stock Purchase Plan		553		665		801
Shares reserved under 2020 Inducement Stock Incentive Plan		1,638		1,857		1,761
Total		19,180		16,793		13,576

Total share-based compensation expense related to the various plans during the years ended was as follows (in thousands):


	Year Ended December 31,
	2023		2022		2021
Research and development	$	45,644	$	42,052	$	30,586
General and administrative		60,301		49,033		40,081
Total share-based compensation expense	$	105,945	$	91,085	$	70,667

Stock Options—Options granted generally vest over 48 months. Options granted to employees on or after December 5, 2013 generally vest in installments of (i) 25% at the one-year anniversary and (ii) in either 36 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial vesting commencement date (as defined) subject to the employee’s continuous service with the Company. Options granted before December 5, 2013 vest over four years in equal annual installments of 25% at each anniversary of the grant date.

Under the Executive Separation Benefits and Retention Plan and by resolutions adopted by the Compensation Committee in October 2019, the stock options granted to the Company’s executives and employees will become fully vested upon the occurrence of a change in control, as defined in the Executive Separation Benefits and Retention Plan, if such executive or employee is terminated without cause or resigns for good reason within 12 months after such change in control.

The following table summarizes the Company’s stock option activity:


				Weighted -		Weighted -
				Average		Average		Aggregate
				Exercise		Contractual		Intrinsic
	Shares			Price		Life		Value
	(in thousands)			Per Share		(in years)		(in thousands)
Outstanding, December 31, 2022		12,295		$	26.31		6.24		317,031
Granted		838			52.48
Exercised		(3,950	)		20.11
Forfeited		(519	)		43.65
Outstanding, December 31, 2023		8,664		$	30.65
Options exercisable, December 31, 2023		6,712		$	26.20		5.39	$	226,419
Expected to vest, December 31, 2023		1,952		$	45.97		8.13	$	27,515

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the common stock as of December 31, 2023.

During the years ended December 31, 2023, 2022 and 2021, the Company granted stock options to purchase an aggregate of 0.8 million, 1.3 million and 2.7 million shares of its common stock, respectively with weighted average grant date fair values of $34.26, $23.62 and $30.72, respectively.

The aggregate intrinsic value of options exercised during the years ended December 31, 2023, 2022 and 2021 were $181.0 million, $44.8 million, and $39.9 million respectively calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options on the respective date of exercise.

The fair market value of options vested during the years ended December 31, 2023, 2022 and 2021 were $46.8 million, $59.0 million and $53.2 million, respectively.

On December 31, 2023, unrecognized compensation expense related to unvested options, was $46.1 million, which the Company expects to recognize over an estimated weighted-average period of 2.18 years.

The assumptions used in the Black-Scholes model to estimate the grant date fair value are as follows:


	Year Ended December 31,
	2023	2022	2021
Risk-free interest rate	3.50 - 4.01%	1.15 - 3.37%	0.41 - 1.34%
Dividend yield	0%	0%	0%
Volatility	68.4 - 71.0%	68.3 - 70.4%	71.7 - 74.4%
Expected terms (years)	3.81 - 6.08	3.81 - 6.08	5.31 - 6.08

Restricted Stock Units— The fair value of RSU’s is estimated based upon the closing market price of the Company’s common stock on the date of grant. RSUs generally vest annually over a four-year period.

The following table summarizes the Company’s RSU's activity:


	Number of Stock Units (in thousands)			Weighted Average Grant Date Fair Value Per Share
Unvested Balance at December 31, 2022		3,572		$	42.23
Granted		2,916			52.19
Vested		(1,015	)		41.52
Forfeited		(1,172	)		48.43
Unvested Balance at December 31, 2023		4,301			47.48

The aggregate intrinsic value of restricted grants vested during the year ended December 31, 2023, was $55.3 million. The fair market value of restricted stock units vested during the year ended December 31, 2023 was $42.2 million.

On December 31, 2023, there was approximately $155.5 million of related unrecognized compensation cost which the Company expects to recognize over a remaining weighted average period of 2.6 years.

Employee Stock Purchase Plan—On December 31, 2023, 552,881 shares of common stock remained available for issuance pursuant to the ESPP. Eligible employees who elect to participate in an offering under the ESPP may have up to 15 percent of their earnings withheld, subject to certain limitations, to purchase shares of common stock pursuant to the ESPP. The price of common stock purchased under the ESPP is equal to 85 percent of the lower of the fair market value of the common stock at the commencement date of each offering period or the relevant purchase date. During the year ended December 31, 2023, a total of 112,064 shares of common stock were issued under the ESPP at average per share price of $48.00. During the year ended December 31, 2023, the Company recorded cash received from the issuance of stock to the ESPP of $5.4 million and recorded $2.1 million of stock-based compensation expense related to the ESPP. No additional shares were reserved to the ESPP in 2023.