The 2015 Equity Incentive Award Plan (the “2015 Plan”) was adopted by Mirna in 2015 and remains active after the Merger, now functioning as the primary equity plan for the Company. The 2015 Plan provides for the granting of a variety of stock‑based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, deferred stock awards, dividend equivalent awards, stock payment awards, performance awards and other stock‑based awards. Pursuant to the evergreen provision of the 2015 Plan, which allows for an annual increase in the number of shares of common stock available for issuance, the Company added 1,613,340 shares to the 2015 Plan on January 1, 2020.

The 2017 Stock Incentive Plan (the “2017 Plan”) was adopted by Private Synlogic in 2017 at the time of the 2017 Reorganization and assumed by the Company during the Merger. The 2017 Plan provides for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards.

The 2015 Employee Stock Purchase Plan (“ESPP”) was adopted by Mirna in 2015 and allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for set offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. The Company suspended the ESPP in 2017. In December 2019, the Board reactivated the ESPP and approved an amendment to the ESPP to (i) reduce the permitted aggregate yearly payroll deduction and maximum number of shares of the Company’s common stock that a participant may purchase per offering period under the ESPP and (ii) establish a period for enrollment for eligible participants. The reactivation of the ESPP was effective immediately. The Company’s executive officers are eligible to participate in the ESPP. There were no options to purchase shares of common stock under the ESPP exercised as of March 31, 2020.

As of March 31, 2020, there were 1,131,108 shares available for future grant under the 2017 Plan and the 2015 Plan, and 375,519 shares available for future grant under the ESPP.

For a full description of the Company’s equity plans, refer to Note 9, Equity-based Compensation and Equity Incentive Plans in the Company’s 2019 Annual Report.

The following table summarizes stock option activity during the three months ended March 31, 2020 under the 2015 Plan and the 2017 Plan.

	Stock options outstanding
						Weighted
				Weighted		average		Aggregate
				average		remaining		Intrinsic
	Number of			exercise		contractual		value^(a)
	options			price		term (in years)		(in thousands)
Outstanding at December 31, 2019		2,286,419		$	9.24		8.7	$	43
Granted		1,052,113			1.77				—
Forfeited		(86,073	)		9.30				1
Outstanding at March 31, 2020		3,252,459			6.82		8.9	$	18

Vested or expected to vest at March 31, 2020		3,252,459			6.82		8.9		18

Exercisable at March 31, 2020		919,285			11.26		8.0	$	—

(a)	The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at March 31, 2020 and December 31, 2019.

As of March 31, 2020, there was $6.9 million of unrecognized share-based compensation related to unvested stock option grants which is expected to be recognized over a weighted average period of 2.4 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur.

The following table shows restricted stock activity during the three months ended March 31, 2020:

	Restricted stock awards
				Grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2019		586,929		$	2.97
Granted		226,335			1.70
Vested		(9,318	)		13.53
Forfeited		(33,755	)		2.45
Unvested at March 31, 2020		770,191		$	2.49

As of March 31, 2020, there was approximately $1.3 million of unrecognized share-based compensation related to restricted stock awards granted, which is expected to be recognized over a weighted average period of 1.9 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur.

The Company has recorded total equity-based compensation expense of approximately $1.1 million during the three months ended March 31, 2020 and $1.0 million and during the three months ended March 31, 2019.

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the three months ended March 31, 2020 and 2019 (in thousands):

	Three Months Ended March 31,
	2020		2019
Research and development	$	441	$	284
General and administrative		654		670
	$	1,095	$	954

The following table summarizes equity‑based compensation expense by type of award for the three months ended March 31, 2020 and 2019 (in thousands):