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.

The 2008 Long Term Incentive Plan (“2008 Plan”) was adopted by Mirna in 2008 and allowed for the grant of incentive stock options to employees and nonqualified stock options and other equity awards to employees and nonemployees. The 2015 Plan is the successor to the 2008 Plan and at the time of the Merger, the remaining awards outstanding thereunder were cancelled and the number of shares with respect to those awards were transferred to the 2015 Plan. As of the Merger, the 2008 Plan was retired.

The 2015 LLC Plan was adopted by Private Synlogic at the time of the 2015 Reorganization, which provided for the grant of equity incentive units to employees, officers, directors or consultants. The 2015 LLC Plan was cancelled pursuant to the 2017 Reorganization as described above.

As of December 31, 2018, there were 477,414 shares available for future grant under the Company’s two active equity incentive plans, the 2017 Plan and the 2015 Plan.

The Company is displaying all equity in its post-Merger amounts, as impacted by the Exchange Ratio.

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

	Year ended December 31,
Employees:	2018			2017
Expected term	6.2 years			6.2 years
Weighted-average, risk-free interest rate		2.8	%		2.1	%
Expected volatility		70.8	%		70.4	%
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, 2017		1,267,221		$	13.62		9.6	$	—
Granted		919,496			9.71
Exercised		(19,830	)		12.28
Forfeited		(394,136	)		12.09
Expired		(32,867	)		13.53
Outstanding at December 31, 2018		1,739,884			11.92		9.0	$	—

Vested or expected to vest at December 31, 2018		1,739,884			11.92		9.0	$	—
Exercisable at December 31, 2018		489,236			13.20		8.4	$	—

During the years ended December 31, 2018 and 2017, 919,496 and 1,281,647 stock options were granted to employees and nonemployees, respectively. Approximately $3.2 million and $2.0 million in equity compensation was recognized related to stock options granted to employees and nonemployees for the years ended December 31, 2018 and 2017, respectively.

The weighted average grant date fair value per share of options granted to employees during the year ended December 31, 2018 was approximately $6.35. The total fair value of awards that vested during the year ended December 31, 2018 was $3.3 million.

As of December 31, 2018, there was approximately $8.4 million of unrecognized share-based compensation related to employees for unvested stock option grants which is expected to be recognized over a weighted average period of 2.6 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. In addition, there was approximately $53,000 of unrecognized share-based compensation, related to unvested stock option grants to nonemployees which is expected to be recognized over a weighted average period of 2.4 years. The amount of equity-based compensation expense related to nonemployees that will ultimately be recorded will depend on the remeasurement of the outstanding awards through their vesting date.

During the year ended December 31, 2018, no shares of restricted common stock were granted. During the year ended December 31, 2017, 1,062,794 shares of restricted common stock were granted, including 1,059,912 shares of restricted common stock (adjusted for the Exchange Ratio) granted in exchange for the restricted common units and incentive units that were cancelled as part of the 2017 Reorganization. These shares retained the same vesting schedule as the cancelled restricted common units and incentive units. Private Synlogic treated these as modifications to the original grants of incentive units because the cancellation and reissuance was deemed to be concurrent. The calculation of the incremental compensation expense was based on the excess of the fair value of the award measured immediately before and after the modification. As a result of the modification, Private Synlogic recognized approximately $26,000 in equity-based compensation.

	Restricted stock awards
				Weighted average grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2016		—		$	—
Awards exchanged upon 2017 Reorganization		1,059,912			13.53
Granted		2,884			19.01
Vested		(671,204	)		13.54
Forfeited		(16,113	)		13.53
Unvested at December 31, 2017		375,479		$	13.55
Granted		—			—
Vested		(186,332	)		13.54
Forfeited		(70,468	)		13.61
Unvested at December 31, 2018		118,679		$	13.54

During the years ended December 31, 2018 and 2017, 186,332 and 671,204 shares of restricted common stock vested and approximately $1.0 and $0.5 million in equity compensation was recognized, respectively.

As of December 31, 2018, there was approximately $0.2 million of unrecognized share-based compensation related to restricted stock awards granted to employees, which is expected to be recognized over a weighted average period of 1.6 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. There was no unrecognized share-based compensation related to unvested restricted stock awards granted to nonemployees at December 31, 2018.

Incentive units issued by Synlogic, LLC under the 2015 LLC Plan generally vested 25% after one year and ratably monthly thereafter over the next 36 months. Certain awards provided for accelerated vesting upon a change in control, as defined in the 2015 LLC Plan. Incentive units did not expire. Holders of incentive units had no voting rights in connection with such incentive units. Each incentive unit was intended to be a profits interest within the meaning of IRS regulations promulgated under the Internal Revenue Code. Each incentive unit had a threshold price, which was the price above which an incentive unit would participate in distributions. In this way, an incentive unit was designed to participate in the future profits and appreciation of Synlogic, LLC. Holders of incentive units would have been entitled to receive profits when and if distributions were in excess of the threshold price of the award set by the Board of Directors on the date of grant.

Synlogic, LLC measured and recorded the value of incentive units granted to nonemployees over the period of time that services were provided and, as such, unvested portions were subject to remeasurement at subsequent reporting periods.

No incentive units were issued during the years ended December 31, 2018 and 2017. In May 2017, all incentive units were cancelled pursuant to the 2017 Reorganization and reissued as restricted common stock. As a result, there was no incentive unit activity during the year ended December 31, 2018 or unrecognized compensation expense related to incentive units as of December 31, 2018

The following table represents a summary of incentive unit activity, as adjusted for the Merger, under the 2015 LLC Plan:

	Incentive units
				Weighted-		Weighted-		Weighted-
				average		average		average
	Number of			strike		threshold		grant date
	units			price		price		fair value
Non-vested units at December 31, 2016		971,906		$	5.22	$	5.93	$	1.01
Granted		—			—		—		—
Vested		(73,719	)		4.01		5.53		0.87
Forfeited		(260,145	)		4.19		5.57		1.05
Non-vested units cancelled upon 2017 Reorganization		(638,042	)		5.78		6.15		1.05
Non-vested units at December 31, 2017		—		$	—	$	—	$	—

Vested or expected to vest at December 31, 2017		—		$	—	$	—	$	—

In May 2017, all restricted common unit awards were cancelled pursuant to the 2017 Reorganization and reissued as restricted common stock. As a result, there was no unrecognized compensation expense related to unvested restricted common units as of December 31, 2018.

The following table shows the restricted common unit activity for the year ended December 31, 2017, prior to the 2017 Reorganization, as adjusted for the Merger:

	Restricted common units
				Grant date
	Number of			fair value
	units			(per unit)
Unvested at December 31, 2016		219,087		$	1.48
Granted		—			—
Vested		(37,770	)		1.48
Forfeited		—			—
Exchanged as part of 2017 Reorganization		(181,317	)		1.48
Unvested at December 31, 2017		—		$	—

The Company has recorded total equity‑based compensation expense of approximately $4.3 million and $2.7 million, during the years ended December 31, 2018 and 2017, respectively. Equity compensation during the years ended December 31, 2018 and 2017 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. Equity compensation during the year ended December 31, 2017 additionally includes equity compensation derived from incentive units and restricted common units granted prior to the 2017 Reorganization.

In July 2015, in connection with the 2015 Reorganization, all outstanding stock options and awards were canceled and reissued as incentive units and restricted common units. As such, equity compensation prior to the Merger was derived from incentive units and from a grant of restricted common units.

In May 2017, in connection with the 2017 Reorganization, the incentive units and restricted common units were cancelled and exchanged for restricted stock awards and stock options. Equity compensation after May 2017, was derived from stock options and restricted stock awards.

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, 2018 and 2017 (in thousands):

The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2018 and 2017 (in thousands):

	Years ended December 31,
	2018		2017
Stock options	$	3,361	$	1,956
Restricted stock awards		956		508
Incentive units		—		132
Restricted common units		—		56
	$	4,317	$	2,652