6. Stock-Based Compensation

The 2016 Equity Incentive Plan (“2016 Plan”) provides for an automatic 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 automatic 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, 2017, an additional 537,233 shares became available for issuance under the 2016 Plan.

The Company granted equity awards under its 2015 Equity Incentive Plan (“2015 Plan”) until April 2016 when it was terminated as to future awards, although it continues to govern the terms of awards that remain outstanding under the 2015 Plan. In connection with the Company’s adoption of the 2016 Plan, all remaining shares available for future issuance under the 2015 Plan were transferred to the 2016 Plan.

During the three months ended June 30, 2017 and 2016, the Company issued an aggregate of 244,100 and 635,800 options to purchase common stock, respectively, under its equity incentive plans for an aggregate fair value of $713,000 and $3.4 million, respectively.

During the six months ended June 30, 2017 and 2016, the Company issued an aggregate of 1,016,900 and 720,217 options to purchase common stock, respectively, under its equity incentive plans for an aggregate fair value of $4.9 million and $3.7 million, respectively.

There were no shares issued and sold under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”) during the three months ended June 30, 2017 and 2016. The Company issued and sold 18,184 shares under the 2016 ESPP for aggregate cash proceeds of $78,000 during the six months ended June 30, 2017 and did not issue or sell any shares during the six months ended June 30, 2016.

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

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2017		2016		2017		2016
Research and development	$	285	$	156	$	395	$	210
General and administrative		381		233		671		319
Total stock-based compensation expense	$	666	$	389	$	1,066	$	529

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						Six Months Ended
	June 30,						June 30,
	2017			2016			2017			2016
Equity Incentive Plans
Expected term		5.96			6.00			5.99			6.00
Expected volatility		86	%		87	%		86	%		87	%
Risk-free interest		1.87	%		1.29	%		2.07	%		1.29	%
Dividend yield		0	%		0	%		0	%		0	%

2016 ESPP
Expected term		—			0.36			0.50			0.36
Expected volatility		—			85	%		79	%		85	%
Risk-free interest		—			0.36	%		0.68	%		0.36	%
Dividend yield		—			0	%		0	%		0	%