In January 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Incentive Award Plan (the “2019 Plan”). The 2019 Plan became effective on February 6, 2019, the day prior to the effectiveness of the registration statement filed in connection with the IPO. Under the 2019 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units, and other stock or cash-based awards to individuals who are then employees, officers, directors or consultants of the Company, and employees and consultants of the Company’s subsidiaries. A total of 5,750,000 shares of common stock were approved to be initially reserved for issuance under the 2019 Plan. The number of shares that remained available for issuance under the 2017 Plan (as defined below) as of the effective date of the 2019 Plan were, and shares subject to outstanding awards under the 2017 Plan as of the effective date of the 2019 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2019 Plan. In addition, the number of shares of common stock available for issuance under the 2019 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2019 Plan, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 5% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of December 31, 2019, an aggregate of 1,751,485 shares of common stock were available for issuance under the 2019 Plan and 4,003,040 shares of common stock were subject to outstanding options under the 2019 Plan.

In January 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Employee Stock Purchase Plan (the “ESPP”). The ESPP became effective as of February 6, 2019, the day prior to the effectiveness of the registration statement filed in connection with the IPO. The ESPP permits participants to purchase common stock through payroll deductions of up to 20% of their eligible compensation. A total of 700,000 shares of common stock were approved to be initially reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be automatically increased on the first day of each calendar year during the first ten-years of the term of the ESPP, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 1% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of December 31, 2019, an aggregate of 700,000 shares of common stock were available for issuance under the ESPP.

The Company’s 2017 Equity Incentive Plan (the “2017 Plan”) permitted the granting of incentive stock options, non-statutory stock options, restricted stock, restricted stock units and other stock-based awards. Subsequent to the adoption of the 2019 Plan, no additional equity awards can be made under the 2017 Plan. As of December 31, 2019, 4,535,020 shares of common stock were subject to outstanding options under the 2017 Plan, and 561,868 shares of restricted stock awards granted under the 2017 plan were unvested.

The fair value of each employee and non-employee stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company, prior to the IPO on February 12, 2019, was a private company and lacked company-specific historical and implied volatility information. Therefore, it estimated its expected volatility based on the historical volatility of a publicly traded set of peer companies. Due to the lack of historical exercise history, the expected term of the Company’s stock options for employees has been determined utilizing the “simplified” method for awards. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is zero based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The following assumptions were used to estimate the fair value of stock option awards granted to employees under the Company’s equity incentive plans and the shares purchasable under the ESPP during the periods presented:

Year Ended December 31,

2019

2018

2017

Employee Stock Options

Expected term (in years)

4.6 - 6.1

5.3 - 6.1

N/A

Risk-free interest rate

1.38% - 2.58%

2.65% - 2.96%

N/A

Volatility

70.25% - 86.92%

69.13% - 77.62%

N/A

Dividend yield

–

N/A

Employee Stock Purchase Plan

Expected term (in years)

0.49 - 1.99

N/A

Risk-free interest rate

1.46% - 1.87%

N/A

Volatility

72.08% - 78.70%

N/A

Dividend yield

–

N/A

The following table summarizes stock option activity for the years ended December 31, 2019 and 2018:

	Shares Subject to Options Outstanding					Weighted- Average
				Weighted-		Remaining
				Average Exercise		Contractual Life		Aggregate
	Shares			Price		(Years)		Intrinsic Value
								(in thousands)
Outstanding as of December 31, 2017		—			—		—	$	—
Options granted		5,143,551		$	7.49
Option exercised		—		$	—
Options forfeited/cancelled		(36,222	)	$	4.85
Outstanding as of December 31, 2018		5,107,329		$	7.51		9.7	$	16,343
Options granted		4,194,624		$	20.11
Option exercised		(419,593	)	$	4.87
Options forfeited/cancelled		(344,300	)	$	11.64
Outstanding as of December 31, 2019		8,538,060		$	13.67		9.0	$	35,385
Options vested and exercisable as of December 31, 2019		1,215,471		$	7.25		8.7	$	10,183

The weighted-average grant date fair value per share for the stock options granted during the year ended December 31, 2019 and 2018 was $13.17 and $4.87, respectively.

The aggregate fair value of stock options that vested during the years ended December 31, 2019 and 2018 was $7.2 million and $0.1 million, respectively.

The aggregate intrinsic value in the above table is calculated as the difference between fair value of the Company’s common stock price and the exercise price of the stock options. The aggregate intrinsic value of stock options exercised during the year ended December 31, 2019 was $6.9 million. There were no stock options exercised during the year ended December 31, 2018.

At December 31, 2019, the total unrecognized compensation related to unvested stock option awards granted was $61.0 million, which the Company expects to recognize over a weighted-average period of approximately 2.9 years.

The summary of the Company’s restricted stock activity during the years ended December 31, 2019, 2018 and 2017 is as follows:

	Number of			Weighted-
	Restricted			Average
	Stock Units			Grant Date
	Outstanding			Fair Value
Nonvested at December 31, 2016		—		$	—
Granted		1,305,421			0.09
Nonvested at December 31, 2017		1,305,421		$	0.09
Granted		8,673,584			5.53
Vested		(2,369,696	)		7.58
Forfeited		(127,277	)		0.09
Nonvested at December 31, 2018		7,482,032		$	4.01
Granted		—			—
Vested		(2,833,506	)		4.05
Forfeited		—			—
Nonvested at December 31, 2019		4,648,526		$	3.98

At December 31, 2019, the total unrecognized compensation related to unvested restricted stock awards granted was $13.5 million, which the Company expects to recognize over a weighted-average period of approximately 2.8 years.

Stock-based compensation expense has been reported in the Company’s consolidated statements of operations as follows (in thousands):

	Year Ended
	December 31,
	2019		2018		2017
Research and development	$	10,227	$	679	$	17
General and administrative		10,592		30,268		15
Total stock-based compensation	$	20,819	$	30,947	$	32

As of December 31, 2019, total unrecognized compensation expense related to the ESPP was $1.3 million, which the Company expects to recognize over a weighted-average period of approximately 1.1 years.