The following table summarizes time-based and performance-based stock compensation expense reflected in the consolidated statements of operations (in thousands):

	Three Months Ended
	March 31
	2021		2020
Research and development	$	6,808	$	11,516
General and administrative		7,871		7,791
Total stock compensation expense	$	14,679	$	19,307

The following table summarizes RSU activity as of March 31, 2021, under the Second Amended and Restated Long Term Incentive Plan (LTIP Plan) agreement:

	Number of RSUs			Weighted-Average Grant Date Fair Value
Outstanding at January 1, 2021		108,551		$	115.54
Granted		260,408			121.46
Vested		(3,302	)		212.12
Forfeited		(12,199	)		128.19
Outstanding at March 31, 2021		353,458		$	118.56

As of March 31, 2021, total unrecognized compensation expense related to RSU awards was approximately $8.1 million, which excludes 291,072 shares of unvested performance-based RSUs that were deemed not probable of vesting totaling unrecognized stock-based compensation expense of $33.1 million.

The following table summarizes stock option activity as of March 31, 2021, under the LTIP Plan and standalone option agreements:

	Number of Options			Weighted- Average Price
Outstanding at January 1, 2021		4,306,269		$	79.47
Granted		697,538			123.39
Exercised		(112,423	)		41.59
Forfeited		(94,271	)		138.30
Expired		(30,727	)		205.84
Outstanding at March 31, 2021		4,766,386		$	84.81
Exercisable at March 31, 2021		2,282,312		$	46.62

Stock-based compensation expense for the three months ended March 31, 2021, included accelerated recognition of expense of $1.4 million due to modifications of outstanding stock options as a result of employees

who entered into consulting agreements at the termination of employment, which were considered to be non-substantive services.

As of March 31, 2021, total unrecognized compensation expense related to stock options was approximately $125.5 million, which excludes 482,800 shares of unvested performance-based stock options that were deemed not probable of vesting totaling unrecognized stock-based compensation expense of $48.8 million.

The total intrinsic value of all outstanding options and exercisable options at March 31, 2021 was $183.7 million and $139.6 million, respectively.