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				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018

Research and development	$	1,670	$	880	$	3,197	$	2,770
General and administrative		2,209		1,068		4,527		4,206
Total	$	3,879	$	1,948	$	7,724	$	6,976

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



	At September 30, 2019
	Unrecognized Compensation Expense		Average Vesting Period
	($ thousands)		(Years)
Stock options grants	$	9,677	2.6
RSU grants		—	—
PRSU grants		1,391	0.9
ESPP		132	0.4

Option Exercises

For the three and nine months ended September 30, 2019, options were exercised to purchase 4,513 and 6,964 shares, respectively, of the Company's common stock, with $13,000 and $21,000 net proceeds, respectively, to the Company in the three and nine months ended September 30, 2019. For each of the three and nine months ended September 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 September 30, 2019, 850,309 of these PRSUs were outstanding. Based on the evaluation of the performance conditions at September 30, 2019, the Company concluded that the PRSU assumptions are probable and recorded a one-time catch-up of stock-based compensation expense of $2.1 million for each of the three and nine months ended September 30, 2019. Stock-based compensation expense recorded related to these PRSUs were zero for the three and nine months ended September 30, 2018. The related compensation cost was recognized as an expense over the estimated vesting period ratably when achievement of the milestone was 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.

For the three and nine months ended September 30, 2019, the Company issued zero and 85,609 shares of its common stock upon vesting of the time-based RSUs to its employees. No PRSUs were issued for the three and nine months ended September 30, 2019. For each of the three and nine months ended September 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 August 2019, the Company sold 77,698 shares of its common stock under the ESPP. The shares were purchased by employees at a purchase price of $2.58 per share with proceeds to the Company of approximately $0.2 million. 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 August 2018, the Company sold 52,370 shares of its common stock under the ESPP. The shares were purchased by employees at a purchase price of $3.66 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 nine months ended September 30, 2019, the Company issued zero and 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 nine months ended September 30, 2018, the Company issued zero and 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.