In the accompanying condensed consolidated statements of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

	Three Months Ended March 31,
	2021		2020
	(in thousands)
Research and development	$	1,663	$	546
General and administrative		1,268		372
Total stock-based compensation	$	2,931	$	918

The fair values of the Company’s stock-based awards granted during the three months ended March 31, 2021 and 2020 were estimated as of the grant date using the Black-Scholes option pricing model, based on assumptions as follows:

	Three Months Ended March 31,
	2021			2020
Expected term (in years)	5.5 – 7.0			5.7 – 6.7
Expected volatility		84 – 85	%		88 – 90	%
Risk-free interest rate		0.5 – 1.1	%		0.6 – 1.2	%
Expected dividend rate		—	%		—	%

The fair value of each stock option grant was determined by the Company on the date of grant 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. As the Company’s historical share option exercise is limited due to a lack of sufficient data points, and does not provide a reasonable basis upon which to estimate an expected term, the expected term is derived by using the midpoint between the weighted-average vesting term and the contractual expiration period of the stock-based award.

Expected Volatility— For the three months ended March 31, 2021, the expected volatility is derived from using the Company’s trading history for its common stock over a period equivalent to the expected term of the stock-based awards. For the three months ended March 31, 2020, the expected volatility was derived from a weighted volatility using both the Company’s trading history for its common stock and the historical stock volatilities of peer public companies within its industry that are considered to be comparable to the Company’s business over a period equivalent to the expected term of the stock-based awards.

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 stock-based awards’ expected term.

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

In September 2018, the Company’s Compensation Committee approved the 2018 Equity Inducement Plan (2018 Plan). The number of shares available for awards under the 2018 Plan was set to 37,500. On June 30, 2020, the Company’s Board of Directors approved an amendment to the 2018 Plan to increase the authorized number of shares available for issuance by 500,000 shares. On February 3, 2021, the Company’s Compensation Committee approved an amendment to the 2018 Plan to increase the authorized number of shares available for issuance by 500,000. As of March 31, 2021, 1,037,500 shares were reserved for issuance under the 2018 Plan. The exercise price of each stock-based award issued under the 2018 Plan is required to be no less than the fair value of the Company’s common stock. The vesting and exercise provisions of options or restricted awards granted are determined individually with each grant. Stock options have a 10-year life and expire if not exercised within that period or if not exercised within three months of cessation of employment with the Company or such longer period of time as specified in the option agreement.

The 2015 Equity Incentive Plan (2015 Plan) became effective on July 14, 2015. On January 21, 2020 the Company’s stockholders approved the following amendments to the 2015 Plan: (i) increase to the authorized number of shares available for issuance by 4,312,500 shares and proportionately increase the share limit related to incentive stock options, (ii) provide limits on the total value of compensation that may be granted to any non-employee director in each calendar year, and (iii) eliminate the annual individual grant limit to reflect changes to the tax law in 2017 tax legislation.

As of March 31, 2021, 5,123,736 shares were reserved for issuance under the 2015 Plan. The number of shares reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31. The Company’s Board of Directors or Compensation Committee may reduce the amount of the increase in any particular year. The exercise price of each stock-based award issued under the 2015 Plan is required to be no less than the fair value of the Company’s common stock. The vesting and exercise provisions of options or restricted awards granted are determined individually with each grant. Stock options have a 10-year life and expire if not exercised within that period or if not exercised within three months of cessation of employment with the Company or such longer period of time as specified in the option agreement, unless modified.

The Company granted options under the 2008 Stock Plan (2008 Plan) until July 2015 when it was terminated as to future awards, although it continues to govern the terms of options that remain outstanding under the 2008 Plan. The 2008 Plan provided for the granting of Incentive Stock Options (ISO), nonqualified stock options and stock purchase rights. In connection with the Board of Director’s approval of the 2015 Plan, all remaining shares available for future award under the 2008 Plan were transferred to the 2015 Plan, and the 2008 Plan was terminated.

A summary of activity under the 2008 Plan, 2015 Plan and 2018 Plan and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Shares Outstanding			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value of Outstanding Options (in thousands)
Outstanding — December 31, 2020		1,117,796			4,146,928		$	19.45		8.84	$	12,227
Awards authorized		945,139
Options granted		(650,460	)		650,460			16.52
Options exercised	-				(21,662	)		13.28
Options forfeited/cancelled		108,553			(108,553	)		14.72
Outstanding — March 31, 2021		1,521,028			4,667,173		$	19.18	8.76		$	16,794
Exercisable — March 31, 2021					755,413		$	46.18	5.66		$	1,905
Vested and expected to vest — March 31, 2021					4,131,935		$	20.09	8.67		$	14,075

The weighted-average grant date fair values of options granted during the three months ended March 31, 2021 and 2020 was $11.73 and $9.81 per share, respectively. The aggregate intrinsic value of options exercised was $0.1 million for the three months ended March 31, 2021. No options were exercised for the three months ended March 31, 2020. The total grant date fair values of options vested for the three months ended March 31, 2021 and 2020 was $3.4 million and $1.6 million, respectively.

In August 2020, the Company granted executives and employees 1,107,250 stock options with performance-based conditions. Vesting is achieved based upon the completion of pre-determined milestones. As of March 31, 2021, all of the performance-based options remain unvested. For the three months ended March 31, 2021, the Company has recognized approximately $0.8 million in stock-based compensation expense related to the options with performance-based criteria. No expense pertaining to performance-based options was recognized for the three months ended March 31, 2020.

As of March 31, 2021, total unrecognized stock-based compensation related to unvested stock options with only service-vesting conditions was $27.7 million and are expected to be recognized over a remaining weighted-average period of 3.2 years. As of March 31, 2021, total unrecognized stock-based compensation related to unvested stock options with performance-based conditions was $7.9 million.