6. Stock-Based Compensation

Stock Plans

2015 Equity Incentive Plan

The 2015 Plan provides for the grant of incentive stock options, or ISOs, non-statutory stock options, or NSOs, restricted stock awards, restricted stock unit awards and stock appreciation rights. In December 2017 and April 2018, our board of directors and our stockholders, respectively, approved an additional amendment to our 2015 Equity Incentive Plan to increase the number of shares of common stock reserved for issuance thereunder to 13,500,000 shares of common stock.

As of June 30, 2018, the aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2015 Plan was 13,500,000 and we had 2,348,332 shares available for future issuance under the 2015 Plan. During the three and six months ended June 30, 2018, we granted to employees and directors no shares of restricted stock and options to purchase 2,392,800 and 4,202,892 shares of common stock, respectively. At June 30, 2018 a total of 11,128,684 options were outstanding, of which 3,682,584 were exercisable, 22,984 restricted stock unit awards were outstanding.

2017 Employee Stock Purchase Plan

The number of shares of common stock that may be issued under the Employee Stock Purchase Plan, or ESPP, will automatically increase commencing on January 1, 2018 and ending on (and including) January 1, 2027 in an amount equal to the lesser of (i) 1% of the total number of shares of Common Stock outstanding on December 31st of the preceding calendar year, and (ii) 500,000 shares of Common Stock. On January 1, 2018, 500,000 shares of common stock were added to the ESPP.

As of June 30, 2018, the aggregate number of shares of common stock reserved under the 2017 ESPP was 1,000,000 and we had 984,268 shares available for future issuance under the 2017 ESPP. During the three and six months ended June 30, 2018, no shares and 15,732 shares, respectively, were issued under our 2017 ESPP. At June 30, 2018, accrued liabilities included $0.2 million of ESPP contributions related to our current enrollment period for which the related shares will be issued on July 2, 2018.

Stock-Based Compensation

The following table summarizes stock-based compensation expense for the three and six months ended June 30, 2018 and 2017 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expenses	$	2,242	$	1,919	$	4,555	$	3,520
General and administrative expenses		9,884		2,023		13,954		3,602
Total	$	12,126	$	3,942	$	18,509	$	7,122