	Year Ended December 31,
	2020		2019		2018
	(In thousands)
Research and development	$	14,980	$	8,031	$	4,195
General and administrative		15,542		8,250		2,730
Total stock-based compensation	$	30,522	$	16,281	$	6,925

The estimated grant-date fair value of all the Company’s options to purchase common stock was calculated using the Black-Scholes option pricing model, based on the following assumptions:

	Year Ended December 31,
	2020	2019	2018
Expected term (in years)	5.3-6.1	5.3 – 6.1	5.1 – 6.1
Expected volatility	74%-78%	76% – 82%	73% – 81%
Risk-free interest rate	0.3% - 1.8%	1.4% – 2.6%	2.5% – 3.1%
Dividend yield	0%	0%	0%

The fair value of each stock option was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management.

Expected Term—The expected term represents the period that stock-based awards are expected to be outstanding. The expected term was derived by using the simplified method which uses the midpoint between the average vesting term and the contractual expiration period of the stock-based award.

Expected Volatility—The Company has limited information on the volatility of stock options as the shares were not actively traded on any public markets prior to February 7, 2019. The expected volatility was derived from the historical stock volatilities of comparable peer public companies within its industry. These companies are considered to be comparable to the Company’s business over a period equivalent to the expected term of the stock-based awards. In 2020, the Company began giving weight to in its own historical volatility in the determination of expected volatility.

Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the expected term.

Expected Dividend Rate—The expected dividend is zero as the Company has not paid nor does it anticipate paying any dividends on its common stock underlying its stock options in the foreseeable future.

The restricted common stock generally vests over a four-year period with straight-line vesting with a 25% one-year cliff.

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted common stock as of December 31, 2018		1,917,848		$	6.95
Vested		(866,037	)		6.95
Forfeited		(56,973	)		6.95
Unvested restricted common stock as of December 31, 2019		994,838			6.95
Vested		(726,659	)		6.95
Forfeited		(81,754	)		6.95
Unvested restricted common stock as of December 31, 2020		186,425			6.95

As of December 31, 2020, total unrecognized stock-based compensation related to unvested restricted common stock issued to employees was $0.9 million, which the Company expects to recognize over a remaining weighted-average period of 0.8 years.

On February 6, 2019, the Company adopted the 2019 Equity Incentive Plan (2019 Plan) under which the Board may issue incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units, and performance shares to the Company’s employees, directors, and consultants. The Company’s 2017 Stock Option and Grant Plan (2017 Plan) was terminated; however, shares subject to awards granted under it will continue to be governed by the 2017 Plan. Shares reserved for issuance but not issued pursuant to, or not subject to, awards granted under the 2017 Plan were added to the available shares in the 2019 Plan. Shares subject to awards granted under the 2017 Plan that are repurchased by, or forfeited to, the Company will also be reserved for issuance under the 2019 Plan. The board of directors, or a committee appointed by the board of directors, has the authority to determine to whom options or shares 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 exercise price of each share shall be at least 110% of the fair market value and the term of the award shall not exceed five years. All other options granted under the 2019 Plan must have an exercise price at least equal to the fair market value on the date of grant and have a term not to exceed ten years. The shares generally vest over a four-year period with straight-line vesting with a 25% one-year cliff. As of December 31, 2020, the Company had reserved 15,628,217 shares of common stock for issuance under the 2019 Plan, of which 2,973,219 shares were available for issuance.

	Number of Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding as of December 31, 2018		5,063,688		$	8.94
Granted		3,738,071			17.74
Exercised		(180,287	)		8.43
Forfeited		(178,648	)		11.52
Outstanding as of December 31, 2019		8,442,824			12.79
Granted		5,597,148			15.35
Exercised		(655,772	)		9.65
Forfeited		(729,202	)		14.96
Outstanding as of December 31, 2020		12,654,998		$	13.96		8.7	$	39,633
Exercisable as of December 31, 2020		3,588,568		$	12.74		8.1	$	13,393
Vested and expected to vest as of December 31, 2020		12,654,998		$	13.96		8.7	$	39,633

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money. The aggregate intrinsic value of options exercised was $11.1 million and $1.8 million for the years ended December 31, 2020 and 2019. There were no options exercised in 2018. The weighted-average grant-date fair value per share of options granted was $10.14, $12.06, and $6.27, for the years ended December 31, 2020, 2019, and 2018, respectively. As of December 31, 2020, total unrecognized stock-based compensation related to unvested stock options was $82.9 million, which the Company expects to recognize over a remaining weighted-average period of 2.6 years.

On February 6, 2019, the Company adopted the 2019 Employee Stock Option Plan (2019 ESPP). The 2019 ESPP will enable eligible employees of the Company to purchase shares of common stock at a discount. Each offering period will be approximately six months long beginning on the first trading day on or after December 1 and June 1 each year. ESPP participants purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the first trading day of the offering period or (2) the fair market value of the common stock on the purchase date. As of December 31, 2020, there was $0.4 million in unrecognized compensation expense related to the 2019 ESPP to be recognized over five months.