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, restricted units, performance shares, and other types of equity awards.

As of December 31, 2020, 1,784,478 shares were registered and available for grant under approved registration statements, while 3,938,285 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 some 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:

	Year ended December 31,
	2020		2019		2018
Estimated fair values of stock options granted	$	3.24	$	10.18	$	9.09
Assumptions:
Risk-free interest rate	.3% - 1.7%		1.7% - 2.5%		2.2% - 3.0%
Expected volatility	77.4% - 81.2%		52.6% - 78.9%		40.9% - 57.2%
Expected term (in years)	6.0 - 10		6.8 - 10		5.6 - 10

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.

Options may be priced at 100 percent or more of the fair market value on the date of grant and generally vest over three to six years after the date of grant. Options generally expire within 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, 2019		1,789,567	$	8.73		4,481,359		$	11.73
Granted						887,765			4.67
Exercised						(58,575	)		3.60
Forfeited or expired						(689,376	)		12.89
Balance as of December 31, 2020		2,347,665	$	10.15		4,621,173		$	10.30

	Year ended December 31,
In Thousands	2020		2019		2018
Stock-based compensation expenses	$	3,371	$	6,035	$	3,609

The aggregate intrinsic value of options exercisable at December 31, 2020 was $0.6 million and the weighted average remaining contractual term was 6.2 years as of that date.

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:

	Year ended December 31,
In Thousands	2020		2019		2018
Net cash proceeds	$	212	$	344	$	2,622
Intrinsic value of options exercised	$	179	$	905	$	7,569

As of December 31, 2020, unrecognized compensation expense related to non-vested stock options was $7.7 million. This expense will be recognized over 30 months on average.

Units settled in stock subject to a restricted period may be granted to key employees 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, 2019		813,526		$	10.31
Granted		105,633			6.54
Vested		(309,693	)		10.08
Cancelled		(61,659	)		10.80
Unvested balance at December 31, 2020		547,807		$	9.49

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

	Year ended December 31,
In Thousands	2020		2019		2018
Grant-date fair value	$	3,122	$	3,141	$	2,691

Information on the weighted average grant date fair value of restricted stock units issued was as follows:

	Year ended December 31,
In Thousands	2020		2019		2018
Weighted average grant date fair value	$	6.54	$	12.48	$	16.76

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

	Year ended December 31,
In Thousands	2020		2019		2018
Stock-based compensation expenses	$	1,155	$	2,910	$	776

As of December 31, 2020, unrecognized compensation expense related to restricted stock units was $2.0 million. This expense will be recognized over 25 months on average.

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

	Year ended December 31,
In Thousands	2020		2019		2018
Deemed dividends from grants to Cellectis employees	$	1,168	$	1,358	$	2,253

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 percent, 100 percent, or 120 percent 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.

The estimated fair values of performance stock units granted, and the assumptions used for the Monte Carlo simulation pricing model were as follows:

Estimated fair values of performance stock units granted	$	7.06
Assumptions:
Risk-free interest rate		1.71	%
Expected volatility		75.0	%
Expected term (in years)	3.0

As of December 31, 2020, unrecognized compensation expense related to performance stock units was $1.5 million and will be recognized over 42 months.

Prior to 2018, Cellectis granted stock options to our employees. Compensation costs related to these grants have been recognized in the statements of operations with a corresponding credit to stockholders’ equity, representing the Cellectis’ capital contribution to us. The fair value of each stock option was estimated at the grant date using the Black-Scholes option pricing model.

We recognized stock-based compensation expense related to our Cellectis’ grants of $0.1 million in 2018. Expenses in 2019 were immaterial and as of December 31, 2019, all expenses related to these awards had been recognized.