Stock Plans (Tables)

12 Months Ended

Dec. 31, 2021

The following table summarizes option activity for the year ended December 31, 2021:

We use the Black-Scholes-Merton option-pricing model with the following assumptions to estimate the stock-based compensation expense:


	Years Ended December 31,
	2021		2020
Weighted-average risk-free interest rate	0.5	%	0.4	%
Expected dividend payments	—		—
Expected holding period (years)(1)	5.45		5.79
Weighted-average volatility factor(2)	1.14		1.04
Estimated forfeiture rates for options granted	30	%	27	%

(1)	Expected holding period is based on historical experience of similar awards, giving consideration to the contractual terms of the stock-based awards, vesting schedules and the expectations of future employee behavior.

(2)	Weighted average volatility is based on the historical volatility of our common stock.

(3)	Estimated forfeiture rates are based on historical data.

The following table summarizes the stock-based compensation expense (in thousands):


	Years Ended December 31,
	2021		2020
Research and development	$	749	$	—
General and administrative		756		7
Total stock-based compensation expenses	$	1,505	$	7