During the three months ended March 31, 2021, the Company granted 370,137 restricted stock units to employees. The restricted stock units had a weighted average grant date fair value of $5.53 per share, of which 202,237 shares had a vesting period of 1 year and 167,900 shares had a vesting period of four years.

As of March 31, 2021, there was $4.1 million of unrecognized stock-based compensation expense related to options and PNQs, which is expected to be recognized over a weighted average period of approximately 1.8 years, and $7.5 million and $4.9million of unrecognized stock-based compensation expense related to RSUs and Market RSUs, respectively, which is expected to be recognized over a weighted average period of approximately 3.1 and 2.2 years, respectively.

Total stock-based compensation expense recognized in the accompanying condensed consolidated statements of operations for the three months ended March 31, 2021 and 2020 was as follows (in thousands):

	Three Months Ended March 31,
	2021		2020
RSUs and options	$	1,826	$	1,066
Employee stock purchase plan		109		62
Total stock compensation expense	$	1,935	$	1,128

The Company provides all employees, including executive officers, the ability to purchase our common stock at a discount under our 2004 employee stock purchase plan (the “ESPP”). The ESPP is designed to comply with Section 423 of the Internal Revenue Code and provides all employees with the opportunity to purchase up to $25,000 worth of our common stock (based on the undiscounted fair market value at the commencement of the offering period) each year at a purchase price that is the lower of 85% of the fair market value of the common stock on either the date of purchase or the commencement of the offering period. An employee may not purchase more than 5,000 shares of common stock on any purchase date. The executives’ rights under the ESPP are identical to those of all other employees.

The Company issued 292,981 and 333,727 shares of common stock pursuant to the ESPP for the three months ended March 31, 2021 and 2020, respectively. There were approximately 1.3 million shares of common stock available for issuance under the ESPP as of March 31, 2021.