6. Stock-Based Compensation

On February 20, 2018, the Board of Directors approved and adopted the 2018 Equity Inducement Plan (“2018 Plan”) which became effective on the same date. The Board of Directors approved an initial reserve of 1,100,000 shares of common stock to be used exclusively for individuals who were not previously employees or directors, or following a bona fide period of non-employment, as an inducement material to the individual entering into employment with the Company. Nonqualified stock options or restricted stock units may be granted under the 2018 Plan at the discretion of the Compensation Committee or the Board of Directors. The Company did not seek stockholder approval of the 2018 Plan pursuant to Nasdaq Rule 5635(c)(4). As of March 31, 2018, no grants had been awarded under the 2018 Plan.

The 2016 Equity Incentive Plan (“2016 Plan”) provides for an annual increase in the number of shares available for issuance thereunder, to be added on the first day of each fiscal year, beginning on January 1, 2017 and continuing through 2023, up to 4% of the outstanding number of shares of the Company’s common stock on the December 31 immediately prior to the date of increase, provided that an increase is only effective if the Company’s board of directors either confirmed the increase or approved the increase of a lesser number of shares prior to January 1 of each relevant year. As a result of this provision, on January 1, 2018 and January 1, 2017, an additional 666,807 and 537,233 shares, respectively, became available for issuance under the 2016 Plan.

During the three months ended March 31, 2018 and 2017, the Company issued an aggregate of 829,500 and 772,800 options to purchase common stock, respectively, under its equity incentive plans for an aggregate fair value of $3.9 million and $4.2 million, respectively.

Under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”), the Company issued and sold 30,937 shares for aggregate cash proceeds of $78,000 during the three months ended March 31, 2018 and 18,184 shares for aggregate cash proceeds of $78,000 during the three months ended March 31, 2017.

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,
	2018		2017
Research and development	$	323	$	110
General and administrative		512		291
Total stock-based compensation expense	$	835	$	401

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 equity incentive plans and the shares purchasable under the 2016 ESPP during the periods presented:

	Three Months Ended
	March 31,
	2018			2017
Equity Incentive Plans
Expected term		6.01			5.99
Expected volatility		85	%		86	%
Risk-free interest		2.72	%		2.13	%
Dividend yield		0	%		0	%

2016 ESPP
Expected term		0.49			0.50
Expected volatility		69	%		79	%
Risk-free interest		2.00	%		0.68	%
Dividend yield		0	%		0	%