In August 2015, the Board of Directors approved the 2015 Equity Incentive Plan (“2015 Plan”). In connection with the Company’s IPO and the effectiveness of the 2018 Award Incentive Plan (“2018 Plan”), the 2015 Plan terminated, and no further awards will be granted under the 2015 Plan. The 92,815 shares of common stock shares that were then unissued and available for future issuance under the 2015 Plan became available under the 2018 Plan.

In September 2018, the Company’s Board of Directors approved the 2018 Plan. Under the 2018 Plan, a total of 2,690,000 shares of our common stock were initially reserved for issuance under the 2018 Plan, plus the number of shares remaining available for future awards under the 2015 Plan, as of the effective date of the 2018 Plan. The number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by 4% of the total number of shares of our stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors. The maximum number of shares that may be issued upon the exercise of stock options under the 2018 Plan is 45,000,000.

Prior to the Company’s IPO, the grant date fair value of the Company’s common stock was determined by the Company’s Board of Directors with the assistance of management and an independent third-party valuation specialist.

Subsequent to the Company’s IPO, the grant date fair value of each share of common stock underlying stock option awards is based on the closing price of our common stock as reported by the Nasdaq Select Global Market on the date of grant of the award.

The Company’s Board of Directors has the authority to determine to whom options will be granted, the number of shares, the term, and the exercise price.

If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting.

In September 2018, the Company’s Board of Directors approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”). The 2018 ESPP also became effective in September 2018. A total of 282,334 shares were initially reserved for issuance under the 2018 ESPP. Additionally, the number of shares of common stock reserved for issuance under the 2018 ESPP will increase automatically each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by the lesser of (1) 1% of the shares of common stock outstanding on December 31 of the preceding calendar year or (2) such lesser number of shares determined by the Company’s Board of Directors. The maximum number of shares that may be issued under the 2018 ESPP is 5,000,000. The offering periods are scheduled to start on the first trading day on or after June 1 or December 1 of each year. Contributions under the 2018 ESPP are limited to a maximum of 15% of an employee’s eligible compensation.

The estimated fair value of stock purchase rights granted under the 2018 ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:

	Year ended			Year ended
	December 31, 2020			December 31, 2019
Expected dividend yield		—	%		—	%
Expected term	0.48 years			0.50 years
Risk-free interest rate		0.8	%		2.2	%
Expected volatility		81.0	%		74.0	%

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:

	Year Ended December 31,
	2020			2019			2018
Expected dividend yield		—	%		—	%		—	%
Expected term	6.01 years			6.01 years			6.04 years
Risk-free interest rate		1.1	%		2.3	%		2.8	%
Expected volatility		73.0	%		83.0	%		88.0	%

Management’s calculations are based on a grant date valuation approach. Using the Black-Scholes model, the weighted-average grant-date fair value of employee stock options granted was $5.39, $8.08, and $6.59 per share during the years ended December 31, 2020, 2019, and 2018, respectively.

	Options Outstanding
	Number of Shares Available for Issuance			Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2019		2,851,883			3,165,961		$	7.83		8.45	$	8,875
Authorized		1,454,553			—		$	—
Granted		(1,822,300	)		1,822,300		$	8.41
Exercised		—			(197,544	)	$	1.06
Cancelled		624,276			(624,276	)	$	9.66
Balance at December 31, 2020		3,108,412			4,166,441		$	8.13		8.15	$	2,092
Vested and exercisable – December 31, 2020					1,941,879		$	6.99		7.52	$	1,874
Vested and expected to vest – December 31, 2020					3,894,070		$	8.06		8.10	$	2,078

For the years ended December 31, 2020, 2019, and 2018, the total intrinsic value of stock option awards exercised was $0.9 million, $3.0 million, and $0.8 million, respectively, determined at the date of option exercise, and the total cash received upon exercise of stock options was not significant for either period. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying stock option awards and the estimated fair value of the common stock on the date of exercise.

As of December 31, 2020, $11.4 million of total cost related to non-vested employee and consultant options is expected to be recognized over a weighted-average period of 2.4 years. The total fair value of shares vested during the year ended December 31, 2020 was $6.8 million.

Stock-based compensation expense and awards granted to non-employees was insignificant for the years ended December 31, 2020, 2019 and 2018.

Total stock-based compensation for all awards granted to employees and consultants and our 2018 ESPP, before taxes, is as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development expenses	$	4,453	$	3,437	$	2,081
General and administrative expenses		2,657		1,862		914
Total	$	7,110	$	5,299	$	2,995