In February 2019 and April 2020, the Company issued pre-funded warrants to purchase the Company’s common stock in underwritten public offerings at the offering price of the common stock, less the $0.0001 per share exercise price of each warrant. The warrants were recorded as a component of stockholders’ equity within additional paid-in capital and have no expiration date. Per the terms of the warrant agreements, the outstanding warrants to purchase shares of common stock may not be exercised if the holder’s ownership of the Company’s common stock would exceed 4.99% (“Maximum Ownership Percentage”), or 9.99% for certain holders. By written notice to the Company, each holder may increase or decrease the Maximum Ownership Percentage to any other percentage (not in excess of 19.99% for the majority of such warrants). The Revised Maximum Ownership Percentage would be effective 61 days after the notice is received by the Company.

As of March 31, 2021, the following pre-funded warrants for common stock were issued and outstanding:

Issue Date	Expiration Date	Exercise Price		Number of Warrants Outstanding
February 8, 2019	None	$	0.0001		3,750,000
April 30, 2020	None	$	0.0001		12,860,328
Total pre-funded warrants					16,610,328

The 2016 Equity Incentive Plan (“2016 Plan”) provides for an automatic increase in the number of shares reserved for issuance thereunder on January 1 of each year for the remaining term of the plan (through 2028) equal to (a) 4.0% of the number of issued and outstanding shares of common stock on December 31 of the immediately preceding year, or (b) a lesser amount as approved by the Company’s board of directors each year. As a result of this provision, on January 1, 2021 and January 1, 2020, an additional 1,918,363 and 1,163,377 shares, respectively, became available for issuance under the 2016 Plan.

As of March 31, 2021, the 2016 Plan had 1,369,443 shares available for future issuance.

During the three months ended March 31, 2021 and 2020, the Company issued an aggregate of 1,667,900 and 1,205,000 options to purchase common stock, respectively, under the Company’s equity incentive plans with a weighted-average grant date fair value of $5.14 and $5.27, respectively.

Under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”), the Company issued and sold 38,008 shares for aggregate cash proceeds of $0.2 million during the three months ended March 31, 2021 and 25,928 shares for aggregate cash proceeds of $0.2 million during the three months ended March 31, 2020.

Total stock-based compensation expense related to the Company’s equity incentive plans and 2016 ESPP was as follows (in thousands):

	Three Months Ended
	March 31,
	2021		2020
Research and development	$	637	$	532
General and administrative		1,120		816
Total stock-based compensation expense	$	1,757	$	1,348

The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company’s equity incentive plans, and the shares purchasable under the 2016 ESPP during the periods presented:

	Three Months Ended
	March 31,
	2021			2020
Equity Incentive Plans
Expected term (in years)		6.01			6.02
Expected volatility		83	%		74	%
Risk-free interest		0.76	%		1.45	%
Dividend yield		0	%		0	%

2016 ESPP
Expected term (in years)		0.49			0.50
Expected volatility		97	%		53	%
Risk-free interest		0.08	%		1.49	%
Dividend yield		0	%		0	%