The fair value of employee and director stock options was estimated at the date of grant using a Black-Scholes option valuation model with the weighted average assumptions below.

	Three Months Ended March 31,
	2019			2018
Expected term (in years)		6.1			6.1
Expected volatility		45.0	%		45.9	%
Risk-free interest rate		2.40	%		2.72	%
Dividend yield		0.0	%		0.0	%

The following table summarizes the total stock-based compensation expense included in the statements of operations and comprehensive loss for all periods presented (in thousands):

	Three Months Ended March 31,
	2019		2018
Cost of revenue	$	88	$	117
Research and development		873		576
Selling, general and administrative		3,454		2,554
Total stock-based compensation expense	$	4,415	$	3,247

As of March 31, 2019, there was total unamortized compensation costs of $13.2 million, net of estimated forfeitures, related to unvested stock options which the Company expects to recognize over a period of approximately 2.2 years, $39.5 million, net of estimated forfeitures, related to unrecognized restricted stock unit (“RSU”) expense, which the Company expects to recognize over a period of 2.6 years, and $0.9 million unrecognized ESPP expense, which the Company will recognize over 0.7 years.

During the three months ended March 31, 2019, the Company granted PRSUs to key executives of the Company. The performance equity program has a 2-year performance period measuring target revenue Compound Annual Growth Rate (“CAGR”) achievement for fiscal year 2020 compared to fiscal year 2018. There is a minimum performance threshold of 75% to earn 50% of target, and a maximum threshold of 125% achieved to earn 200% of target. The exact number of earned shares will be determined based on linear interpolation using the revenue CAGR as it falls between the minimum and maximum thresholds outlined above. The fair value of the PRSUs granted during the three months ended March 31, 2019 will range from $0 to $19.5 million, depending on the achievement of the performance target. A total of 202,738 PRSUs were granted and the Company did not record any expense for these grants for the three months ended March 31, 2019. In the event that achievement of the performance targets becomes probable, the Company will recognize a cumulative expense amount for prior periods.