In September 2014, the Board of Directors adopted the 2014 Stock Plan (the 2014 Plan). In June 2015, the Board of Directors adopted the 2015 Equity Incentive Plan (the 2015 Plan), which became effective upon the Company’s initial public offering in September 2015. The 2015 Plan replaced the 2014 Plan, and as of the effective date of the 2015 Plan, no further awards may be issued under the 2014 Plan. Any options or awards outstanding under the 2014 Plan as of the effective date of the 2015 Plan remained outstanding and effective. The number of authorized shares under the 2015 Plan automatically increases annually on the first business day of each fiscal year, by the lesser of (i) 4% of the total number of shares of common stock outstanding on December 31 of the prior year, or (ii) a number of common shares determined by the Board of Directors. As of December 31, 2022, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Plan was 15,625,200, of which 2,796,739 remained available for future grants under the 2015 Plan. An additional 1,731,940 shares were authorized for issuance under the 2015 Plan effective in January 2023.

The 2014 Plan and 2015 Plan provide for the issuance of stock options, stock appreciation rights, restricted and unrestricted stock and unit awards, and performance cash awards to employees, members of the Board of Directors and consultants of the Company. Since the inception of the plans, the Company has issued only stock options and restricted stock units under the plans. Stock options under the 2014 Plan and 2015 Plan generally expire 10 years following the date of grant. Options typically vest over a four-year period, but vesting provisions can vary by award based on the discretion of the Board of Directors. Certain stock option awards granted by the Company may include performance conditions that must be achieved in order for vesting to occur. Stock options under the 2014 Plan and 2015 Plan have an exercise price at least equal to the estimated fair value of the Company’s common

stock on the date of grant. Restricted stock units typically vest over a four-year period, but vesting provisions can vary by award based on the discretion of the Board of Directors. Upon vesting, restricted stock units are settled in common stock of the Company.

Shares of common stock underlying awards previously issued under the 2014 Plan and 2015 Plan which are reacquired by the Company, withheld by the Company in payment of the purchase price, exercise price or withholding taxes, expired, cancelled due to forfeiture or otherwise terminated other than by exercise or settlement, are added to the number of shares of common stock available for issuance under the 2015 Plan. Shares available for issuance under the 2015 Plan may be either authorized but unissued shares of the Company’s common stock or common stock reacquired by the Company and held in treasury. The 2015 Plan expires in June 2025, 10 years from the date it was adopted by the Board of Directors, unless earlier terminated.

The Company’s stock-based compensation expense by award type was as follows (in thousands):


	Years Ended December 31,
	2022		2021		2020
Stock options	$	34,444	$	35,320	$	31,178
Restricted stock units		5,569		2,835		—
Employee stock purchase plan		775		653		771
	$	40,788	$	38,808	$	31,949

As of December 31, 2022, the Company had $63.2 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.3 years.

The Company recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands):


	Years Ended December 31,
	2022		2021		2020
Research and development	$	21,368	$	19,602	$	16,280
General and administrative		19,420		19,206		15,669
	$	40,788	$	38,808	$	31,949

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):


						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2021		7,126		$	34.16		6.8	$	41,128
Granted		1,492		$	32.31
Exercised		(332	)	$	8.45
Cancelled or forfeited		(618	)	$	40.25
Outstanding at December 31, 2022		7,668		$	34.43		6.5	$	15,783
Exercisable at December 31, 2022		5,244		$	33.43		5.6	$	15,756
Vested and expected to vest at December 31, 2022		7,668		$	34.43		6.5	$	15,783

(a)

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

The weighted-average grant date fair value per share of options granted during the years ended December 31, 2022, 2021 and 2020 was $19.43, $26.01 and $23.82, respectively. During the years ended December 31, 2022, 2021 and 2020, the total number of stock options exercised was 331,912, 404,263 and 434,534, respectively, resulting in total proceeds of $2.8 million, $4.3 million and $5.6 million, respectively. The total intrinsic value of options exercised during the years ended December 31, 2022, 2021 and 2020 was $6.9 million, $11.9 million and $11.5 million, respectively.

The fair values of options granted were estimated at each grant date using the Black-Scholes valuation model with the following weighted-average assumptions:


	Years Ended December 31,
	2022			2021			2020
Expected volatility		66	%		68	%		71	%
Expected term (years)		6.0			6.0			6.0
Risk-free interest rate		1.7	%		0.6	%		1.4	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data):


				Weighted-average
				Grant Date
	Shares			Fair Value
Unvested balance at December 31, 2021		277		$	43.62
Granted		444		$	31.71
Vested		(68	)	$	43.78
Forfeited		(40	)	$	36.97
Unvested balance at December 31, 2022		613		$	35.41

The total intrinsic value of restricted stock units vested during the year ended December 31, 2022 was $2.2 million. No restricted stock units vested during the years ended December 31, 2021 and 2020.

In June 2015, the Board of Directors adopted the 2015 ESPP, which became effective upon the Company’s initial public offering in September 2015. The number of authorized shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year by the lesser of (i) 1% of the shares of common stock outstanding on the last business day of the prior fiscal year; or (ii) the number of shares determined by the Board of Directors. Unless otherwise determined by the administrator of the 2015 ESPP, two offering periods of six months’ duration will begin each year on January 1 and July 1. As of December 31, 2022, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 1,426,994, of which 1,121,752 remained available for future issuance. During the years ended December 31, 2022, 2021 and 2020, 75,733, 53,596 and 55,499 shares of common stock, respectively, were issued under the 2015 ESPP.