NOTE 7. STOCK-BASED COMPENSATION

The following table presents stock-based compensation expense recognized for stock options, restricted stock units, or RSUs, performance-based restricted stock units, or PRSUs, and the Company’s employee stock purchase program, or ESPP, in the Company’s statements of operations (in thousands):


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017

Research and development	$	983	$	1,414	$	1,890	$	2,445
General and administrative		1,620		1,376		3,138		2,454
Total	$	2,603	$	2,790	$	5,028	$	4,899

In January 2017, the Company granted PRSU awards to certain employees which vest upon the achievement of specified performance conditions, subject to the employees’ continued service relationship with the Company. None vested during the three and six months ended June 30, 2018 and 2017. However, the related compensation cost is recognized as an expense over the estimated vesting period when achievement of the milestone is considered probable. The expense recognized for these awards is based on the grant date fair value of the Company’s common stock multiplied by the number of units granted. The Company recognized $0.3 million and $0.4 million of related expense during the three and six months ended June 30, 2018, respectively.

At June 30, 2018, the Company had $14.5 million, $1.6 million, $0.3 million and $0.1 million of total unrecognized compensation expense, net of estimated forfeitures, related to stock option grants, RSU grants, PRSUs and the ESPP, respectively, that will be recognized over an average vesting period of 2.7 years, 1.0 years, 0.3 years and 0.2 years, respectively.

Option Exercises

For the three and six months ended June 30, 2018, zero options were exercised to purchase shares of the Company's common stock, with zero net proceeds to the Company. For the three and six months ended June 30, 2017, 9,025 and 28,326 options, respectively, were exercised to purchase shares of the Company’s common stock, with insignificant net proceeds to the Company.

Restricted Stock Units

For the three and six months ended June 30, 2018, the Company issued zero shares, of its common stock upon vesting of restricted stock units to its employees. For the three and six months ended June 30, 2017, the Company issued zero and 15,188 shares, respectively, of the Company’s common stock due to vesting of restricted stock units resulting in insignificant net proceeds to the Company.

Employee Stock Purchase Plan

In February 2018, the Company sold 68,589 shares under the ESPP. The shares were purchased by employees at a purchase price of $4.38 per share with proceeds to the Company of approximately $0.3 million. In February 2017, the Company sold 42,845 shares under the ESPP. The shares were purchased by employees at a purchase price of $8.77 per share with proceeds to the Company of approximately $0.4 million.

Issuance of Common Stock for Services

For the three and six months ended June 30, 2018, the Company issued 75,183 shares of common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under the Non-Employee Director Compensation Plan. The shares issued were valued at $0.3 million based on the fair value of the common stock on the date of grant. For the three and six months ended June 30, 2017, the Company issued 46,858 shares of common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under the Non-Employee Director Compensation Plan. The shares issued were valued at $0.2 million based on the fair value of the common stock on the date of grant.