The 2015 Equity Incentive Award Plan (“2015 Plan”) was adopted by Mirna in 2015 and remains active after the Merger, now functioning as the primary equity plan for the Company. Following the Merger, there were 647,893 shares authorized under the 2015 Plan. The 2015 Plan includes an “evergreen provision” that allows for an annual increase in the number of shares of common stock available for issuance under the 2015 Plan, which annual increase will be added on the first day of each fiscal year from 2016 through 2025, inclusive, and will be equal to the lesser of (i) five percent of the shares outstanding on the last day of the immediately preceding fiscal year and (ii) such smaller number of shares as determined by the Board of Directors. 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.

The 2017 Stock Incentive Plan was adopted by Private Synlogic in 2017 at the time of the 2017 Reorganization and provides for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards. Under the 2017 Plan, 1,753,061 shares were initially authorized and reserved for issuance. Pursuant to the 2017 Reorganization, Private Synlogic issued restricted common stock awards under the 2017 Plan to replace the canceled incentive units pursuant to the termination of the 2015 LLC Plan (“2015 LLC Plan”). In addition, Private Synlogic also issued stock options to certain employees prior to the Merger. Pursuant to the Merger Agreement, each restricted common stock award of Private Synlogic under the 2017 Plan that was outstanding immediately prior to the Merger and each option to purchase common stock of Private Synlogic under the 2017 Plan that was outstanding and unexercised immediately prior to the Merger was converted into and became restricted common stock and options to purchase shares of the Company’s common stock, respectively, based on the Exchange Ratio of 0.5532 and the Company assumed the 2017 Plan.

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. On December 19, 2019, the board of directors (the “Board”) of the Company resolved by written consent to reactivate the 2015 ESPP and approved an amendment to the ESPP to (i) reduce the permitted payroll deduction and number of shares of the Company’s common stock that a participant may purchase per calendar year and offering period under the ESPP and (ii) establish a period for enrollment for eligible participants. The reactivation of the 2015 ESPP was effective immediately. The Company’s executive officers are eligible to participate in the 2015 ESPP.

As of December 31, 2019, there were 676,388 shares available for future grant under the Company’s two active equity incentive plans, the 2017 Plan and the 2015 Plan.

The weighted average assumptions used in the Black-Scholes option-pricing model for stock options issued to employees and non-employees under its two active equity plans, the 2015 Plan and the 2017 Plan, during the years ended December 31, 2019 and 2018 were:

	Year ended December 31,
Employees:	2019			2018
Expected term	6.2 years			6.2 years
Weighted-average, risk-free interest rate		2.2	%		2.8	%
Expected volatility		71.1	%		70.8	%
Dividend yield		—			—

The following table summarizes stock option activity, as adjusted for the Exchange Ratio under the 2015 and 2017 Plans.

	Stock options outstanding
						Weighted
						average
				Weighted		remaining		Aggregate
				average		contractual		intrinsic
	Number of			exercise		term		value^(a)
	options			price		(in years)		(in thousands)
Outstanding at December 31, 2018		1,739,884		$	11.92		9.0	$	—
Granted		1,427,030			7.07				—
Cancelled or Forfeited		(880,495	)		11.02				25
Outstanding at December 31, 2019		2,286,419			9.24		8.7	$	43

Vested or expected to vest at December 31, 2019		2,286,419			9.24		8.7	$	43
Exercisable at December 31, 2019		845,102			11.43		8.1	$	—

The weighted average grant date fair value per share of options granted during the years ended December 31, 2019 and 2018 was approximately $4.59 and $6.35, respectively. The total fair value of awards that vested during the years ended December 31, 2019 and 2018 was $3.9 million and $3.3 million, respectively.

As of December 31, 2019, there was approximately $6.8 million of unrecognized share-based compensation for 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.

During the year ended December 31, 2019, 585,600 shares of restricted common stock were granted. During the year ended December 31, 2018, no shares of restricted common stock were granted.

	Restricted stock awards
				Weighted average grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2018		118,679		$	13.54
Granted		585,600			2.49
Vested		(56,314	)		13.55
Forfeited		(61,036	)		9.19
Unvested at December 31, 2019		586,929		$	2.97

The total fair value of shares that vested during the years ended December 31, 2019 and 2018 was $0.1 million and $1.3 million, respectively.

As of December 31, 2019, there was approximately $1.2 million of unrecognized share-based compensation related to restricted stock awards granted, which is expected to be recognized over a weighted average period of 1.2 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 $4.1 million and $4.3 million, during the years ended December 31, 2019 and 2018, respectively. Equity compensation during the years ended December 31, 2019 and 2018 is derived from stock options and restricted stock awards. Equity-based compensation during the year ended December 31, 2018 also includes $0.7 million related to modifications in equity awards in connection with the separation of the Company’s former Chief Executive Officer.

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the years ended December 31, 2019 and 2018 (in thousands):

The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2019 and 2018 (in thousands):