The Company has two equity incentive plans that provide for the granting of stock options, restricted stock, stock appreciation rights, stock units, and performance cash awards to certain employees, members of the board of directors, and consultants of the Company with a generally prescribed contractual term of ten years. As of March 31, 2021, there were 3,947,976 shares of common stock available for grant under the Company’s equity incentive plans.

In 2019 and 2020 the Company granted performance cash awards. The bonus units vest in four annual installments from the date of grant and entitle the employees to receive a cash payment, on the earlier of (i) four years from the date of grant or (ii) a change of control, equal in value to the amount by which the then value of the Company’s common stock exceeds the base value. As of March 31, 2021, $1.4 million was accrued as compensation expense for vested performance cash awards.

The Company recognizes stock-based compensation expense over the requisite service period. The Company’s share-based awards are accounted for as equity instruments. The amounts included in the consolidated statements of operations relating to stock-based compensation associated with the two equity incentive plans and Helio founders’ shares are as follows:

	Three Months Ended March 31,
	2021		2020
Research and development expenses	$	1,192,867	$	945,738
General and administrative expenses		1,188,789		1,007,914
Total stock-based compensation expense	$	2,381,656	$	1,953,652

The table below summarizes activity relating to stock options under the incentive plans for the three months ended March 31, 2021:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term		Aggregate Intrinsic Value(a)
Outstanding at December 31, 2020		4,608,311		$	5.73
Granted		108,115		$	12.24
Exercised		(543,826	)	$	7.36
Forfeitures		(316,396	)	$	5.23
Outstanding at March 31, 2021		3,856,204		$	5.72		6.87	$	23,774,646
Exercisable at March 31, 2021		2,547,247		$	5.65		5.92	$	15,863,925

As of March 31, 2021, unamortized stock-based compensation for all stock options outstanding was $4,523,182 and is expected to be recognized over a weighted average period of 2.36 years. Total unrecognized compensation cost will be adjusted for future forfeitures, if necessary.

The table below summarizes activity relating to RSUs for the three months ended March 31, 2021:

	Number of Shares
Outstanding at December 31, 2020		927,189
Granted		—
Cancelled		(185,610	)
Vested		(286,944	)
Outstanding at March 31, 2021		454,635

There were no RSUs granted during the three months ended March 31, 2021. As of March 31, 2021, the outstanding RSUs had unamortized stock-based compensation of $2.2 million with a weighted-average remaining recognition period of 2.15 years and an aggregate intrinsic value of $5.4 million.

At March 31, 2021, the Company had 1,242,863 shares available for issuance under the 2016 Employee Stock Purchase Plan (“2016 ESPP”). A summary of the weighted-average grant-date fair value, and total stock-based compensation expense recognized related to the 2016 ESPP are as follows:

	Three Months Ended March 31,
	2021		2020
Weighted-average grant-date fair value per share	$	2.49	$	2.24
Total stock-based compensation expense	$	12,404	$	12,332