NOTE 9. 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,
	2019		2018		2019		2018

Research and development	$	767	$	983	$	1,527	$	1,890
General and administrative		1,156		1,620		2,318		3,138
Total	$	1,923	$	2,603	$	3,845	$	5,028

At June 30, 2019, the Company had total unrecognized stock-based compensation expense, net of estimated forfeitures, of the following:



	At June 30, 2019
	Unrecognized Compensation Expense		Average Vesting Period
	($ thousands)		(Years)
Stock options grants	$	11,208	3.0
RSU grants		—	—
PRSU grants		—	—
ESPP		29	0.2

Option Exercises

For the three and six months ended June 30, 2019, options were exercised to purchase 1,681 and 2,451 shares, respectively, of the Company's common stock, with insignificant net proceeds to the Company in the three and six months ended June 30, 2019. For each of 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.

Restricted Stock Units

In July 2018, the Company granted 903,374 PRSUs to its employees that vest upon the achievement of certain performance conditions, subject to the employees’ continued service relationship with the Company through the achievement date. At June 30, 2019, 861,989 of these PRSUs were outstanding. Based on the evaluation of the performance conditions at June 30, 2019, the Company has not recorded stock-based compensation expense for the three and six months ended June 30, 2019 related to these PRSUs. The Company will continue to evaluate the performance conditions for these PRSUs at each reporting period and will record compensation expense related to the PRSUs accordingly.

For each of the three and six months ended June 30, 2019, the Company issued 85,609 shares of its common stock upon vesting of RSUs to its employees. No PRSUs were issued for the three and six months ended June 30, 2019. For each of the three and six months ended June 30, 2018, the Company issued zero shares of its common stock upon vesting of RSUs or PRSUs to its employees.

Employee Stock Purchase Plan

In February 2019, the Company sold 83,046 shares of its common stock under the ESPP. The shares were purchased by employees at a purchase price of $2.39 per share with proceeds to the Company of approximately $0.2 million. In February 2018, the Company sold 68,589 shares of its common stock 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.

Issuance of Common Stock for Services

For the three and six months ended June 30, 2019, the Company issued 113,136 shares of its common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under the Company’s Amended and Restated Non-Employee Director Compensation Program. 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, 2018, the Company issued 75,183 shares of its common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under the Company’s Amended and Restated Non-Employee Director Compensation Program. The shares issued were valued at $0.3 million based on the fair value of the common stock on the date of grant.