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, 2020 and January 1, 2019, an additional 1,163,377 and 965,603 shares, respectively, became available for issuance under the 2016 Plan.

As of March 31, 2020, the 2016 Plan had 1,511,409 shares available for future issuance.

During the three months ended March 31, 2020 and 2019, the Company issued an aggregate of 1,205,000 and 1,042,300 options to purchase common stock, respectively, under the Company’s equity incentive plans for an aggregate fair value of $6.4 million and $6.2 million, respectively.

Under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”), the Company issued and sold 25,928 shares for aggregate cash proceeds of $0.2 million during the three months ended March 31, 2020 and 20,132 shares for aggregate cash proceeds of $0.1 million during the three months ended March 31, 2019.

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,
	2020		2019
Research and development	$	532	$	402
General and administrative		816		679
Total stock-based compensation expense	$	1,348	$	1,081

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,
	2020			2019
Equity Incentive Plans
Expected term (in years)		6.02			6.03
Expected volatility		74	%		82	%
Risk-free interest		1.45	%		2.57	%
Dividend yield		0	%		0	%

2016 ESPP
Expected term (in years)		0.50			0.49
Expected volatility		53	%		66	%
Risk-free interest		1.49	%		2.55	%
Dividend yield		0	%		0	%