We use broad-based stock plans to attract and retain highly qualified officers and employees and to help ensure that management’s interests are aligned with those of our shareholders. We have also granted equity-based awards to directors, nonemployees, and certain employees of Cellectis.

In December 2014, we adopted the Calyxt, Inc. Equity Incentive Plan (2014 Plan), which allowed for the grant of stock options, and in June 2017, we adopted the 2017 Omnibus Plan (2017 Plan), which allowed for the grant of stock options, restricted stock units, performance stock units and other types of equity awards.

On February 19, 2021, James Blome ceased serving as our Chief Executive Officer. We recorded a benefit to earnings from a $2.5 million recapture of non-cash stock compensation expense from the forfeiture of certain of Mr. Blome’s unvested stock options, restricted stock units, and performance stock units.

As of March 31, 2021, 1,902,128 shares were registered and available for grant under effective registration statements, while 6,202,032 shares were available for grant in the form of stock options, restricted stock, restricted stock units, and performance stock units under the 2017 Plan. Stock-based awards currently outstanding also include awards granted under the 2014 Plan, under which no further awards will be granted.

The estimated fair values of stock options granted, and the assumptions used for the Black-Scholes option pricing model were as follows:

	Three Months Ended March 31,
	2021			2020
Estimated fair values of stock options granted	$	5.85		$	5.19
Assumptions:
Risk-free interest rate		0.6	%		1.7	%
Expected volatility		85.0	%		77.4	%
Expected term (in years)	5.7 - 6.2				6.9

We estimate the fair value of each option on the grant date, or other measurement dates if applicable, using a Black-Scholes option-pricing model, which requires us to make predictive assumptions regarding employee exercise behavior, future stock price volatility, and dividend yield. Our expected term represents the period that options granted are expected to be outstanding determined using the simplified method. We estimate our future stock price volatility using the historical volatility of comparable public companies over the expected term of the option. We estimate the risk-free interest rate based on the United States Treasury zero-coupon yield curve at the date of grant for the expected term of the option. We do not nor do we expect to pay dividends.

Option strike prices are set at 100 percent or more of the closing share price on the date of grant, and generally vest over three to six years following the grant date. Options generally expire 10 years after the date of grant.

	Options Exercisable		Weighted- Average Exercise Price Per Share		Options Outstanding			Weighted- Average Exercise Price Per Share
Balance as of December 31, 2020		2,347,663	$	10.15		4,621,173		$	10.30
Granted						270,800			8.07
Exercised						(56,372	)		3.71
Forfeited or expired						(456,450	)		10.03
Balance as of March 31, 2021		2,369,997	$	10.38		4,379,151		$	10.27

As of March 31, 2021, options outstanding and exercisable had an aggregate intrinsic value of $2.2 million and the weighted average remaining contractual term was 6.0 years.

Net cash proceeds from the exercise of stock options less shares used for minimum withholding taxes and the intrinsic value of options exercised were as follows:

	Three Months Ended March 31,
In Thousands	2021		2020
Net cash proceeds	$	208	$	—
Intrinsic value of options exercised	$	331	$	—

As of March 31, 2021, unrecognized compensation expense related to non-vested stock options was $6.9 million. This expense will be recognized over 30 months on average.

Units settled in stock subject to a restricted period may be granted under the 2017 Plan. Restricted stock units generally vest and become unrestricted over three to five years after the date of grant.

	Number of Restricted Stock Units Outstanding			Weighted- Average Grant Date Fair Value
Unvested balance at December 31, 2020		547,807		$	9.49
Granted		68,000			8.05
Vested		(27,386	)		9.18
Forfeited		(126,178	)		12.89
Unvested balance at March 31, 2021		462,243		$	8.37

The total grant-date fair value of restricted stock unit awards that vested is as follows:

Stock-based compensation expense related to restricted stock units is as follows:

As of March 31, 2021, unrecognized compensation expense related to restricted stock units was $1.7 million. This expense will be recognized over 27 months on average.

We treat stock-based compensation awards granted to employees of Cellectis as deemed dividends. We recorded deemed dividends as follows:

In June 2019, we granted 311,667 performance stock units under the 2017 Plan to three executive officers. The performance stock units will vest at 50%, 100% or 120% of the shares under the award at the end of a three-year performance period based upon increases in the value of our common stock from the grant price of $12.48. The performance stock units will be settled in restricted stock upon vesting, with restrictions on transfer lapsing on the second anniversary of the restricted stock issuance date. During the three months ended March 31, 2021, we recognized a benefit from the forfeiture of 166,667 performance stock units held by Mr. Blome, our former chief executive officer.

Stock-based compensation expense related to performance stock units is as follows:

As of March 31, 2021, unrecognized compensation expense related to performance stock units was $0.7 million. This expense will be recognized over 39 months on average.