The following table summarizes our stock option activity under our stock plans and related information:

	Options Outstanding			Weighted- Average Exercise Price
Balance at December 31, 2019		3,887,780		$	18.38
Options granted		1,279,300		$	67.08
Options forfeited		(8,000	)	$	30.03
Options exercised		(72,833	)	$	6.89
Balance at March 31, 2020		5,086,247		$	30.78
Vested and expected to vest		5,086,247		$	30.78
Exercisable as of March 31, 2020		2,483,500		$	11.12

At March 31, 2020 and December 31, 2019, 21,925 shares and 25,081 shares, respectively, related to the early exercise of stock options, were subject to repurchase by us at the original exercise price in the event the optionee’s employment is terminated either voluntarily or involuntarily. We classify cash received from the exercise of unvested options as a short-term liability. These liabilities were approximately $0.2 million as of March 31, 2020 and December 31, 2019, and were included in accrued liabilities on the condensed consolidated balance sheets. Amounts from such liabilities are reclassified into common stock and additional paid-in capital as the shares vest, which is generally over 48 months.

The following table summarizes total stock-based compensation expense related to our 2018 Equity Incentive Plan, 2008 Equity Incentive Plan and 2018 Employee Stock Purchase Plan (in thousands):

	Three Months Ended March 31,
	2020		2019
Research and development	$	2,343	$	1,150
General and administrative		2,046		1,055
Total	$	4,389	$	2,205

As of March 31, 2020, there was approximately $82.1 million of unamortized compensation cost related to stock option awards that is expected to be recognized as expense over a weighted-average period of approximately 3.4 years. The weighted-average remaining contractual term of options outstanding at March 31, 2020 was 8.1 years.