2012 Stock Option/Stock Issuance Plan of the Registrant

Exhibit 10.43

CARIBOU BIOSCIENCES, INC.

2012 STOCK OPTION/STOCK ISSUANCE PLAN

ARTICLE ONE

GENERAL PROVISIONS

This 2012 Stock Option/Stock Issuance Plan is intended to promote the interests of Caribou Biosciences, Inc., a Delaware corporation, by providing eligible persons in the Corporation’s employ or service with the opportunity to acquire a proprietary interest, or otherwise increase their proprietary interest, in the Corporation as an incentive for them to continue in such employ or service.

Capitalized terms herein shall have the meanings assigned to such terms in the attached Appendix.

A. The Plan shall be divided into two (2) separate equity programs:

(i) the Option Grant Program under which eligible persons may, at the discretion of the Plan Administrator, be granted options to purchase shares of Common Stock, and

(ii) the Stock Issuance Program under which eligible persons may, at the discretion of the Plan Administrator, be issued shares of Common Stock directly, either through the immediate purchase of such shares or as a bonus for services rendered the Corporation (or any Parent or Subsidiary) or pursuant to restricted stock units or other share right awards which vest upon the completion of designated service periods or the attainment of pre-established performance milestones.

B. The provisions of Articles One and Four shall apply to both equity programs under the Plan and shall accordingly govern the interests of all persons under the Plan.

A. The Plan shall be administered by the Board. However, any or all administrative functions otherwise exercisable by the Board may be delegated to the Committee. Members of the Committee shall serve for such period of time as the Board may determine and shall be subject to removal by the Board at any time. The Board may also at any time terminate the functions of the Committee and reassume all powers and authority previously delegated to the Committee.

B. The Plan Administrator shall have full power and authority (subject to the provisions of the Plan) to establish such rules and regulations as it may deem appropriate for proper administration of the Plan and to make such determinations under, and issue such interpretations of, the Plan and any outstanding options or stock issuances thereunder as it may deem necessary or advisable. Decisions of the Plan Administrator shall be final and binding on all parties who have an interest in the Plan or any option grant or stock issuance thereunder.

A. The persons eligible to participate in the Plan are as follows:

(i) Employees,

(ii) non-employee members of the Board or the non-employee members of the board of directors of any Parent or Subsidiary, and

(iii) consultants and other independent advisors who provide services to the Corporation (or any Parent or Subsidiary).

B. The Plan Administrator shall have full authority to determine, (i) with respect to the grants made under the Option Grant Program, which eligible persons are to receive such grants, the time or times when those grants are to be made, the number of shares to be covered by each such grant, the status of the granted option as either an Incentive Option or a Non-Statutory Option, the time or times when each option is to become exercisable, the vesting schedule (if any) applicable to the option shares and the maximum term for which the option is to remain outstanding, and (ii) with respect to stock issuances or other stock-based awards under the Stock Issuance Program, which eligible persons are to receive such issuances or awards, the time or times when those issuances or awards are to be made, the number of shares subject to each such issuance or award, the applicable vesting schedule and the cash consideration (if any) to be paid by the Participant for such shares.

C. The Plan Administrator shall have the absolute discretion either to grant options in accordance with the Option Grant Program or to effect stock issuances in accordance with the Stock Issuance Program.

A. The stock issuable under the Plan shall be shares of authorized but unissued or reacquired Common Stock. The maximum number of shares of Common Stock which may be issued over the term of the Plan shall not exceed 650,000 shares.

B. Shares of Common Stock subject to outstanding options, restricted stock units or share right awards shall be available for subsequent issuance under the Plan to the extent (i) those options, units or awards expire, terminate or are cancelled for any reason prior to the issuance of the underlying shares of Common Stock or (ii) such options are cancelled in accordance with the cancellation-regrant provisions of Article Two. Unvested shares issued than the option exercise or direct issue price paid per share, pursuant to the Corporation’s repurchase rights under the Plan shall be added back to the number of shares of Common Stock reserved for issuance under the Plan and shall accordingly be available for reissuance through one or more subsequent option grants or direct stock issuances under the Plan.

C. Should any change be made to the Common Stock by reason of any stock split, stock dividend, spin-off transaction, extraordinary distribution (whether in cash, securities or other property), recapitalization, combination of shares, exchange of shares or other similar transaction affecting the outstanding Common Stock without the Corporation’s receipt of consideration or in the event of a substantial reduction to the value of the outstanding shares of Common Stock by reason of a spin-off transaction or extraordinary distribution or in the event of any merger, consolidation or other reorganization, then equitable adjustments shall be made to (i) the maximum number and/or class of securities issuable under the Plan, (ii) the number and/or class of securities and the exercise price per share in effect under each outstanding option, (iii) the number and/or class of securities subject to each outstanding restricted stock unit or other share right award and the cash consideration (if any) payable per share and (iv) the number and/or class of securities subject to the Corporation’s outstanding repurchase rights under the Plan and the repurchase price payable per share. In the event of a Change in Control, the provisions of Article Two, Section III shall apply. The adjustments shall be made by the Plan Administrator in such manner as the Plan Administrator deems appropriate, and those adjustments shall be final, binding and conclusive. In no event shall any such adjustments be made in connection with the conversion of one or more outstanding shares of the Corporation’s preferred stock into shares of Common Stock.

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ARTICLE TWO

OPTION GRANT PROGRAM

Each option shall be evidenced by one or more documents in the form approved by the Plan Administrator; provided, however, that each such document shall comply with the terms specified below. Each document evidencing an Incentive Option shall, in addition, be subject to the provisions of the Plan applicable to such options.

A. Exercise Price.

1. The exercise price per share shall be fixed by the Plan Administrator but shall not be less than one hundred percent (100%) of the Fair Market Value per share of Common Stock on the option grant date.

2. The exercise price shall become immediately due upon exercise of the option and shall, subject to the provisions of Section I of Article Four and the documents evidencing the option, be payable in cash or check made payable to the Corporation. Should the Common Stock be registered under Section 12 of the 1934 Act at the time the option is exercised, then the exercise price may also be paid as follows:

(i) in shares of Common Stock valued at Fair Market Value on the Exercise Date and held for the period (if any) necessary to avoid a charge to the Corporation’s earnings for financial reporting purposes, or

(ii) to the extent the option is exercised for vested shares, through a special sale and remittance procedure pursuant to which the Optionee shall concurrently provide irrevocable instructions (A) to a brokerage firm (reasonably satisfactory to the Corporation for purposes of administering such procedure in compliance with any applicable pre-clearance or pre-notification requirements) to effect the immediate sale of the purchased shares and remit to the Corporation, out of the sale proceeds available on the settlement date, sufficient funds to cover the aggregate exercise price payable for the purchased shares plus all applicable income and employment taxes required to be withheld by the Corporation by reason of such exercise and (B) to the Corporation to deliver the certificates for the purchased shares directly to such brokerage firm on the settlement date in order to complete the sale.

Except to the extent such sale and remittance procedure is utilized, payment of the exercise price for the purchased shares must be made on the Exercise Date.

B. Exercise and Term of Options. Each option shall be exercisable at such time or times, during such period and for such number of shares as shall be determined by the Plan Administrator and set forth in the documents evidencing the option grant. However, no option shall have a term in excess of ten (10) years measured from the option grant date.

C. Effect of Termination of Service.

1. The following provisions shall govern the exercise of any options held by the Optionee at the time of cessation of Service or death:

(i) Should the Optionee cease to remain in Service for any reason other than death, Disability or Misconduct, then the Optionee shall have a period of three (3) months from the date of such cessation of Service during which to exercise each outstanding option held by such Optionee.

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(ii) Should the Optionee’s Service terminate by reason of Disability, then the Optionee shall have a period of twelve (12) months from the date of such cessation of Service during which to exercise each outstanding option held by such Optionee.

(iii) If the Optionee dies while holding an outstanding option, then the personal representative of his or her estate or the person or persons to whom the option is transferred pursuant to the Optionee’s will or the laws of inheritance or the Optionee’s designated beneficiary or beneficiaries of that option shall have a twelve (12)-month period from the date of the Optionee’s death to exercise such option.

(iv) Under no circumstances, however, shall any such option be exercisable after the specified expiration of the option term.

(v) During the applicable post-Service exercise period, the option may not be exercised in the aggregate for more than the number of vested shares for which the option is at the time exercisable. No additional shares shall vest under the option following the Optionee’s cessation of Service, except to the extent (if any) specifically authorized by the Plan Administrator in its sole discretion pursuant to an express written agreement with the Optionee. Upon the expiration of the applicable exercise period or (if earlier) upon the expiration of the option term, the option shall terminate and cease to be outstanding.

(vi) Should the Optionee’s Service be terminated for Misconduct or should the Optionee otherwise engage in Misconduct while holding one or more outstanding options under the Plan, then all those options shall terminate immediately and cease to remain outstanding.

2. The Plan Administrator shall have the discretion, exercisable either at the time an option is granted or at any time while the option remains outstanding, to:

(i) extend the period of time for which the option is to remain exercisable following the Optionee’s cessation of Service or death from the limited period otherwise in effect for that option to such greater period of time as the Plan Administrator shall deem appropriate, but in no event beyond the expiration of the option term, and/or

(ii) permit the option to be exercised, during the applicable post-Service exercise period, not only with respect to the number of vested shares of Common Stock for which such option is exercisable at the time of the Optionee’s cessation of Service but also with respect to one or more additional installments in which the Optionee would have vested under the option had the Optionee continued in Service.

D. Stockholder Rights. The holder of an option shall have no stockholder rights with respect to the shares subject to the option until such person shall have exercised the option, paid the exercise price and become the recordholder of the purchased shares.

E. Unvested Shares. The Plan Administrator shall have the discretion to grant options which are exercisable for unvested shares of Common Stock. Should the Optionee cease Service while holding such unvested shares, the Corporation shall have the right to repurchase any or all of those unvested shares at a price per share equal to the lower of (i) the exercise price paid per share or (ii) the Fair Market Value per share of Common Stock at the time of the Optionee’s cessation of Service. The terms upon which such repurchase right shall be exercisable (including the period and procedure for exercise and the appropriate vesting schedule for the purchased shares) shall be established by the Plan Administrator and set forth in the document evidencing such repurchase right.

F. First Refusal Rights. Until such time as the Common Stock is first registered under Section 12 of the 1934 Act, the Corporation shall have the right of first refusal with respect to any proposed disposition by the Optionee (or any successor in interest) of any shares of Common Stock issued under the Plan. Such right of first refusal shall be exercisable in accordance with the terms established by the Plan Administrator and set forth in the document evidencing such right.

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G. Limited Transferability of Options.

1. An Incentive Stock Option shall be exercisable only by the Optionee during his or her lifetime, and such Incentive Stock Option, together with the shares of Common Stock subject to that option during the period prior to exercise, shall not be assignable or transferable other than by will or by the laws of inheritance following the Optionee’s death.

2. A Non-Statutory Option, together with the shares of Common Stock subject to that option during the period prior to exercise, shall be subject to the same transfer restrictions as set forth in subparagraph 1 above, except that a Non-Statutory Option, together with the underlying unexercised shares of Common Stock, may to the extent permitted by the Plan Administrator be assigned in whole or in part during the Optionee’s lifetime by gift or pursuant to a domestic relations order to one or more of the Optionee’s Family Members or to a trust established exclusively for the Optionee and/or one or more such Family Members. The assigned portion may only be exercised by the person or persons who acquire a proprietary interest in the Non-Statutory Option pursuant to the assignment. The terms applicable to the assigned portion shall be the same as those in effect for the option immediately prior to such assignment and shall be set forth in such documents issued to the assignee as the Plan Administrator may deem appropriate.

3. Notwithstanding subparagraphs 1 and 2 above, the Optionee may also, to the extent permitted by the Plan Administrator, designate one or more Family Members as the beneficiary or beneficiaries of his or her outstanding options under the Plan, and those options shall, in accordance with such designation, automatically be transferred to such beneficiary or beneficiaries upon the Optionee’s death while holding those options. Such beneficiary or beneficiaries shall take the transferred options subject to all the terms and conditions of the applicable agreement evidencing each such transferred option, including (without limitation) the limited time period during which the option may be exercised following the Optionee’s death.

4. Prior to the date the Corporation first becomes subject to the reporting requirements of Section 13 or 15(d) of the 1934 Act, outstanding options under the Plan, together with the shares of Common Stock subject to those options during the period prior to exercise, shall not be the subject of any short position, put equivalent position (as such term is defined in Rule 16a-1(h) under the 1934 Act) or call equivalent position (as such term is defined Rule 16a-1(b) of the 1934 Act).

5. Except as otherwise provided in subparagraph 1, 2 or 3 above, until the date the Corporation first becomes subject to the reporting requirements of Section 13 or 15(d) of the 1934 Act, outstanding options under the Plan, together with the shares of Common Stock subject to those options during the period prior to exercise, shall not be the subject of any pledges, gifts, hypothecations or other transfers, other than pursuant to the Corporation’s repurchase rights or in connection with a Change in Control of the Corporation in which such options shall terminate and cease to be outstanding.

The terms specified below shall be applicable to all Incentive Options. Except as modified by the provisions of this Section II, all the provisions of Articles One, Two and Four shall be applicable to Incentive Options. Options which are specifically designated as Non-Statutory Options shall not be subject to the terms of this Section II.

A. Eligibility. Incentive Options may only be granted to Employees.

B. Dollar Limitation. The aggregate Fair Market Value of the shares of Common Stock (determined as of the respective date or dates of grant) for which one or more options granted to any Employee under the Plan (or any other option plan of the Corporation or any Parent or Subsidiary) may for the first time become exercisable as Incentive Options during any one (1) calendar year shall not exceed the sum of One Hundred Thousand

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Dollars ($100,000). To the extent the Employee holds two (2) or more such options which become exercisable for the first time in the same calendar year, the foregoing limitation on the exercisability of such options as Incentive Options shall be applied on the basis of the order in which such options are granted, except to the extent otherwise provided under applicable law or regulation.

C. 10% Stockholder. If any Employee to whom an Incentive Option is granted is a 10% Stockholder, then the option term shall not exceed five (5) years measured from the option grant date, and the exercise price per share shall not be less than one hundred ten percent (110%) of the Fair Market Value per share of Common Stock on the option grant date.

A. The shares subject to each option outstanding under the Plan at the time of a Change in Control shall automatically vest in full so that each such option shall, immediately prior to the effective date of the Change in Control, become exercisable for all of the shares of Common Stock at the time subject to that option and may be exercised for any or all of those shares as fully-vested shares of Common Stock. However, the shares subject to an outstanding option shall not vest on such an accelerated basis if and to the extent: (i) such option is assumed by the successor corporation (or parent thereof) or otherwise continued in full force and effect pursuant to the terms of the Change in Control transaction and any repurchase rights of the Corporation with respect to any unvested shares purchasable under the option are concurrently assigned to such successor corporation (or parent thereof) or otherwise continued in effect or (ii) such option is to be replaced with a cash retention program of the Corporation or any successor corporation which preserves the spread existing on the unvested option shares at the time of the Change in Control (the excess of the Fair Market Value of those shares over the aggregate exercise price payable for such shares) and provides for subsequent payout of that spread in accordance with the same vesting schedule applicable to those unvested option shares but only if such replacement cash program would not result in the treatment of the Award as an item of deferred compensation subject to Code Section 409A or (iii) the acceleration of such option is subject to other limitations imposed by the Plan Administrator at the time of the option grant.

B. All outstanding repurchase rights shall also terminate automatically, and the shares of Common Stock subject to those terminated rights shall immediately vest in full, in the event of any Change in Control, except to the extent: (i) those repurchase rights are assigned to the successor corporation (or parent thereof) or otherwise continued in full force and effect pursuant to the terms of the Change in Control transaction or (ii) such accelerated vesting is precluded by other limitations imposed by the Plan Administrator at the time the repurchase right is issued.

C. Immediately following the consummation of the Change in Control, all outstanding options shall terminate and cease to be outstanding, except to the extent assumed by the successor corporation (or parent thereof) or otherwise continued in effect pursuant to the terms of the Change in Control transaction.

D. Each option which is assumed in connection with a Change in Control or otherwise continued in effect shall be appropriately adjusted, immediately after such Change in Control, to apply to the number and class of securities which would have been issuable to the Optionee in consummation of such Change in Control, had the option been exercised immediately prior to such Change in Control. Appropriate adjustments shall also be made to (i) the number and class of securities available for issuance under the Plan following the consummation of such Change in Control and (ii) the exercise price payable per share under each outstanding option, provided the aggregate exercise price payable for such securities shall remain the same. To the extent the actual holders of the Corporation’s outstanding Common Stock receive cash consideration for their Common Stock in consummation of the Change in Control, the successor corporation may, in connection with the assumption or continuation of the outstanding options under this Plan, substitute one or more shares of its own common stock with a fair market value equivalent to the cash consideration paid per share of Common Stock in such Change in Control.

E. The Plan Administrator shall have the discretion, exercisable either at the time the option is granted or at any time while the option remains outstanding, to structure one or more options so that those options shall automatically accelerate and vest in full (and any repurchase rights of the Corporation with respect to the unvested shares subject to those options shall immediately terminate) upon the occurrence of a Change in Control, whether or not those options are to be assumed in the Change in Control or otherwise continued in effect.

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F. The Plan Administrator shall also have full power and authority, exercisable either at the time the option is granted or at any time while the option remains outstanding, to structure such option so that the shares subject to that option will automatically vest on an accelerated basis should the Optionee’s Service terminate by reason of an Involuntary Termination within a designated period (not to exceed eighteen (18) months) following the effective date of any Change in Control in which the option is assumed or otherwise continued in effect and the repurchase rights applicable to those shares do not otherwise terminate. Any option so accelerated shall remain exercisable for the fully-vested option shares until the expiration or sooner termination of the option term. In addition, the Plan Administrator may provide that one or more of the Corporation’s outstanding repurchase rights with respect to shares held by the Optionee at the time of such Involuntary Termination shall immediately terminate on an accelerated basis, and the shares subject to those terminated rights shall accordingly vest at that time.

G. The portion of any Incentive Option accelerated in connection with a Change in Control shall remain exercisable as an Incentive Option only to the extent the applicable One Hundred Thousand Dollar ($100,000) limitation is not exceeded. To the extent such dollar limitation is exceeded, the accelerated portion of such option shall be exercisable as a Non-Statutory Option under the Federal tax laws.

H. The grant of options under the Plan shall in no way affect the right of the Corporation to adjust, reclassify, reorganize or otherwise change its capital or business structure or to merge, consolidate, dissolve, liquidate or sell or transfer all or any part of its business or assets.

The Plan Administrator shall have the authority to effect, at any time and from time to time, with the consent of the affected option holders, the cancellation of any or all outstanding options under the Plan and to grant in substitution therefor new options covering the same or different number of shares of Common Stock but with an exercise price per share based on the Fair Market Value per share of Common Stock on the new option grant date.

In the event there are at any time five hundred (500) or more holders of outstanding options under the Plan, the Corporation shall provide to each such option holder, at the time the outstanding options first become held by five hundred (500) holders and at successive six (6)-month intervals thereafter, financial statements that meet the requirements of Rule 701(e)(4) under the 1933 Act and that are at the time of distribution not more than one hundred and eighty (180) days old. Such obligation shall continue until such time as the Corporation becomes subject to the reporting requirements of Section 13 or 15(d) of the 1934 Act or (if earlier) no longer relies on the exemption from such reporting requirements provided by Rule 12h-1(g) under the 1934 Act.

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ARTICLE THREE

STOCK ISSUANCE PROGRAM

Shares of Common Stock may be issued under the Stock Issuance Program through direct and immediate issuances without any intervening option grants. Each such stock issuance shall be evidenced by a Stock Issuance Agreement which complies with the terms specified below. Shares of Common Stock may also be issued under the Stock Issuance Program pursuant to share right awards or restricted stock units.

A. Issue Price.

1. The issue price per share shall be fixed by the Plan Administrator but shall not be less than one hundred percent (100%) of the Fair Market Value per share of Common Stock on the issue date.

2. Subject to the provisions of Section I of Article Four, shares of Common Stock may be issued under the Stock Issuance Program for any of the following items of consideration which the Plan Administrator may deem appropriate in each individual instance:

(i) cash or check made payable to the Corporation,

(ii) past services rendered to the Corporation (or any Parent or Subsidiary), or

(iii) any other valid consideration under the Delaware General Corporation Law.

B. Vesting Provisions.

1. Shares of Common Stock issued under the Stock Issuance Program may, in the discretion of the Plan Administrator, be fully and immediately vested upon issuance or may vest in one or more installments over the Participant’s period of Service or upon attainment of specified performance objectives. Shares of Common Stock may also be issued under the Stock Issuance Program pursuant to share right awards or restricted stock units which entitle the recipients to receive the shares underlying those awards or units upon the attainment of designated performance goals or the satisfaction of specified Service requirements or upon the expiration of a designated time period following the vesting of those awards or units, including (without limitation) a deferred distribution date following the termination of the Participant’s Service.

2. Any new, substituted or additional securities or other property (including money paid other than as a regular cash dividend) which the Participant may have the right to receive with respect to the Participant’s unvested shares of Common Stock by reason of any stock dividend, stock split, spin-off transaction, extraordinary distribution (whether in cash, securities or other property), recapitalization, combination of shares, exchange of shares or other similar change affecting the outstanding Common Stock as a class without the Corporation’s receipt of consideration shall be issued subject to (i) the same vesting requirements applicable to the Participant’s unvested shares of Common Stock and (ii) such escrow arrangements as the Plan Administrator shall deem appropriate.

3. The Participant shall have full stockholder rights with respect to any shares of Common Stock issued to the Participant under the Stock Issuance Program, whether or not the Participant’s interest in those shares is vested. Accordingly, the Participant shall have the right to vote such shares and to receive any regular cash dividends paid on such shares. The Participant shall not have any stockholder rights with respect to the shares of Common Stock subject to a share right award or restricted stock unit until that award or unit vests and the shares of Common Stock are actually issued thereunder. However, dividend-equivalent units may be paid or credited, either in cash or in actual or phantom shares of Common Stock, on outstanding share right awards or restricted stock units, subject to such terms and conditions as the Plan Administrator may deem appropriate.

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4. Should the Participant cease to remain in Service while holding one or more unvested shares of Common Stock issued under the Stock Issuance Program or should the performance objectives not be attained with respect to one or more such unvested shares of Common Stock, then those shares shall be immediately surrendered to the Corporation for cancellation, and the Participant shall have no further stockholder rights with respect to those shares. To the extent the surrendered shares were previously issued to the Participant for consideration paid in cash or cash equivalent (including the Participant’s purchase-money indebtedness), the Corporation shall repay to the Participant the lower of (i) the cash consideration paid for the surrendered shares or (ii) the Fair Market Value of those shares at the time of Participant’s cessation of Service and shall cancel the unpaid principal balance of any outstanding purchase-money note of the Participant attributable to such surrendered shares by the applicable clause (i) or (ii) amount.

5. The Plan Administrator may in its discretion waive the surrender and cancellation of one or more unvested shares of Common Stock (or other assets attributable thereto) which would otherwise occur upon the non-completion of the vesting schedule applicable to those shares. Such waiver shall result in the immediate vesting of the Participant’s interest in the shares of Common Stock as to which the waiver applies. Such waiver may be effected at any time, whether before or after the Participant’s cessation of Service or the attainment or non-attainment of the applicable performance objectives.

6. Outstanding share right awards or restricted stock units shall automatically terminate, and no shares of Common Stock shall actually be issued in satisfaction of those awards or units, if the performance goals or Service requirements established for such awards or units are not attained or satisfied. The Plan Administrator, however, shall have the discretionary authority to issue vested shares of Common Stock under one or more outstanding share right awards or restricted stock units as to which the designated performance goals or Service requirements have not been attained or satisfied.

C. First Refusal Rights. Until such time as the Common Stock is first registered under Section 12 of the 1934 Act, the Corporation shall have the right of first refusal with respect to any proposed disposition by the Participant (or any successor in interest) of any shares of Common Stock issued under the Stock Issuance Program. Such right of first refusal shall be exercisable in accordance with the terms established by the Plan Administrator and set forth in the document evidencing such right.

A. Upon the occurrence of a Change in Control, all outstanding repurchase rights under the Stock Issuance Program shall terminate automatically, and the shares of Common Stock subject to those terminated rights shall immediately vest in full, except to the extent: (i) those repurchase rights are assigned to the successor corporation (or parent thereof) or otherwise continued in full force and effect pursuant to the terms of the Change in Control transaction or (ii) such accelerated vesting is precluded by other limitations imposed by the Plan Administrator at the time the repurchase right is issued.

B. Each share right award or restricted stock unit outstanding at the time of a Change in Control may be (i) assumed by the successor corporation (or parent thereof) or otherwise continued in effect pursuant to the terms of such Change in Control transaction or (ii) replaced with a cash incentive program of the successor program which preserves the Fair Market Value of the Common Stock underlying that award or unit at the time of the Change in Control and provides for subsequent payout of that dollar amount in accordance with the vesting schedule in effect for such award or unit at the time of the Change in Control. If any such share right award or restricted stock unit is not so assumed or otherwise continued in effect, or if such award or unit is not replaced with a cash retention award, then such award or unit shall vest, and the shares of Common Stock subject to such award or unit shall become issuable, immediately prior to the consummation of the Change in Control.

C. Each share right award or restricted stock unit which is so assumed or otherwise continued in effect shall be adjusted immediately after the consummation of that Change in Control so as to apply to the number and class of securities into which the shares of Common Stock subject to the award or unit immediately prior to the

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Change in Control would have been converted in consummation of such Change in Control had those shares actually been outstanding at that time. Appropriate adjustments shall also be made to the cash consideration (if any) price payable per share under each outstanding share right award or restricted stock unit, provided the aggregate cash consideration payable for such securities shall remain the same. To the extent the actual holders of the Corporation’s outstanding Common Stock receive cash consideration for their Common Stock in consummation of the Change in Control, the successor corporation may, in connection with the assumption or continuation of the outstanding share right awards or restricted stock units, substitute one or more shares of its own common stock with a fair market value equivalent to the cash consideration paid per share of Common Stock in such Change in Control transaction.

D. The Plan Administrator shall have the discretionary authority to structure one or more unvested stock issuances or one or more share right awards or restricted stock units under the Stock Issuance Program so that the shares of Common Stock subject to those issuances or awards or units shall automatically vest (or vest and become issuable) in whole or in part immediately upon the occurrence of a Change in Control or upon the subsequent termination of the Participant’s Service by reason of an Involuntary Termination within a designated period (not to exceed eighteen (18) months) following the effective date of that Change in Control transaction.

Unvested shares may, in the Plan Administrator’s discretion, be held in escrow by the Corporation until the Participant’s interest in such shares vests or may be issued directly to the Participant with restrictive legends on the certificates evidencing those unvested shares.

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ARTICLE FOUR

MISCELLANEOUS

The Plan Administrator may permit any Optionee or Participant to pay the option exercise price under the Option Grant Program or the purchase price for shares issued under the Stock Issuance Program by delivering a full-recourse promissory note payable in one or more installments which bears interest at a market rate and is secured by the purchased shares. In no event, however, may the maximum credit available to the Optionee or Participant exceed the sum of (i) the aggregate option exercise price or purchase price payable for the purchased shares plus (ii) any applicable income and employment tax liability incurred by the Optionee or the Participant in connection with the option exercise or share purchase.

A. The Plan shall become effective when adopted by the Board, but no option granted under the Plan may be exercised, and no shares shall be issued under the Plan, until the Plan is approved by the Corporation’s stockholders. If such stockholder approval is not obtained within twelve (12) months after the date of the Board’s adoption of the Plan, then all options previously granted under the Plan shall terminate and cease to be outstanding, and no further options shall be granted and no shares shall be issued under the Plan. Subject to such limitation, the Plan Administrator may grant options and issue shares under the Plan at any time after the effective date of the Plan and before the date fixed herein for termination of the Plan.

B. The Plan shall terminate upon the earliest of (i) the expiration of the ten (10)-year period measured from the date the Plan is adopted by the Board, (ii) the date on which all shares available for issuance under the Plan shall have been issued as vested shares or (iii) the termination of all outstanding awards under the Plan in connection with a Change in Control. All awards and unvested stock issuances outstanding at the time of a clause (i) termination event shall continue to have full force and effect in accordance with the provisions of the documents evidencing those awards or issuances.

A. The Board shall have complete and exclusive power and authority to amend or modify the Plan in any or all respects. However, no such amendment or modification shall adversely affect the rights and obligations with respect to awards or unvested stock issuances at the time outstanding under the Plan unless the Optionee or the Participant consents to such amendment or modification. In addition, certain amendments may require stockholder approval pursuant to applicable laws and regulations.

B. Options may be granted under the Option Grant Program and awards and shares may be issued under the Stock Issuance Program which are in each instance in excess of the number of shares of Common Stock then available for issuance under the Plan, provided any excess shares actually issued under those programs shall be held in escrow until there is obtained stockholder approval of an amendment sufficiently increasing the number of shares of Common Stock available for issuance under the Plan. If such stockholder approval is not obtained within twelve (12) months after the date the first such excess grants or issuances are made, then (i) any unexercised options granted on the basis of such excess shares shall terminate and cease to be outstanding and (ii) the Corporation shall promptly refund to the Optionees and the Participants the exercise or purchase price paid for any excess shares issued under the Plan and held in escrow, together with interest (at the applicable Short Term Federal Rate) for the period the shares were held in escrow, and such shares shall thereupon be automatically cancelled and cease to be outstanding.

Any cash proceeds received by the Corporation from the sale of shares of Common Stock under the Plan shall be used for general corporate purposes.

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The Corporation’s obligation to deliver shares of Common Stock upon the exercise of any options granted under the Plan or upon the issuance or vesting of any shares issued under the Plan shall be subject to the satisfaction of all applicable income and employment tax withholding requirements.

The implementation of the Plan, the granting of any options under the Plan and the issuance of any shares of Common Stock (i) upon the exercise of any option or (ii) under the Stock Issuance Program shall be subject to the Corporation’s procurement of all approvals and permits required by regulatory authorities having jurisdiction over the Plan, the options granted under it and the shares of Common Stock issued pursuant to it.

Nothing in the Plan shall confer upon the Optionee or the Participant any right to continue in Service for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Corporation (or any Parent or Subsidiary employing or retaining such person) or of the Optionee or the Participant, which rights are hereby expressly reserved by each, to terminate such person’s Service at any time for any reason, with or without cause.

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APPENDIX

The following definitions shall be in effect under the Plan:

A. Board shall mean the Corporation’s Board of Directors.

B. Change in Control shall mean a change in ownership or control of the Corporation effected through any of the following transactions:

(i) a merger, consolidation or other reorganization approved by the Corporation’s stockholders, unless securities representing more than fifty percent (50%) of the total combined voting power of the voting securities of the successor corporation are immediately thereafter beneficially owned, directly or indirectly and in substantially the same proportion, by the persons who beneficially owned the Corporation’s outstanding voting securities immediately prior to such transaction, or

(ii) a stockholder-approved sale, transfer or other disposition of all or substantially all of the Corporation’s assets in liquidation or dissolution of the Corporation, or

(iii) the acquisition, directly or indirectly by any person or related group of persons (other than the Corporation or a person that directly or indirectly controls, is controlled by, or is under common control with, the Corporation), of beneficial ownership (within the meaning of Rule 13d-3 of the 1934 Act) of securities possessing more than fifty percent (50%) of the total combined voting power of the Corporation’s outstanding securities pursuant to a tender or exchange offer made directly to the Corporation’s stockholders.

In no event shall any public offering of the Corporation’s securities be deemed to constitute a Change in Control.

C. Code shall mean the Internal Revenue Code of 1986, as amended.

D. Committee shall mean a committee of one (1) or more Board members appointed by the Board to exercise one or more administrative functions under the Plan.

E. Common Stock shall mean the Corporation’s common stock.

F. Corporation shall mean Caribou Biosciences, Inc., a Delaware corporation, and any successor corporation to all or substantially all of the assets or voting stock of Caribou Biosciences, Inc. which shall by appropriate action adopt the Plan.

G. Disability shall mean the inability of the Optionee or Participant to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment and shall be determined by the Plan Administrator on the basis of such medical evidence as the Plan Administrator deems warranted under the circumstances.

H. Employee shall mean an individual who is in the employ of the Corporation (or any Parent or Subsidiary), subject to the control and direction of the employer entity as to both the work to be performed and the manner and method of performance.

I. Exercise Date shall mean the date on which the Corporation shall have received written notice of the option exercise.

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J. Fair Market Value per share of Common Stock on any relevant date shall be determined in accordance with the following provisions:

(i) If the Common Stock is at the time traded on the Nasdaq Global or Global Select Market, then the Fair Market Value shall be the closing selling price per share of Common Stock on the date in question, as such price is reported by the National Association of Securities Dealers for that particular Stock Exchange and published in The Wall Street Journal. If there is no closing selling price for the Common Stock on the date in question, then the Fair Market Value shall be the closing selling price on the last preceding date for which such quotation exists.

(ii) If the Common Stock is at the time listed on any other Stock Exchange, then the Fair Market Value shall be the closing selling price per share of Common Stock on the date in question on the Stock Exchange determined by the Plan Administrator to be the primary market for the Common Stock, as such price is officially quoted in the composite tape of transactions on such exchange and published in The Wall Street Journal. If there is no closing selling price for the Common Stock on the date in question, then the Fair Market Value shall be the closing selling price on the last preceding date for which such quotation exists.

(iii) If the Common Stock is not at the time listed on any Stock Exchange, then the Fair Market Value shall be determined by the Plan Administrator through the reasonable application of a reasonable valuation method that takes into account the applicable valuation factors set forth in the Treasury Regulations issued under Section 409A of the Code; provided, however, that with respect to an Incentive Option, such Fair Market Value shall be determined in accordance with the standards of Section 422 of the Code and the applicable Treasury Regulations thereunder.

K. Family Member means, with respect to a particular Optionee or Participant, any child, stepchild, grandchild, parent, stepparent, grandparent, spouse, former spouse, sibling, niece, nephew, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law.

L. Incentive Option shall mean an option which satisfies the requirements of Code Section 422.

M. Involuntary Termination shall mean the termination of the Service of any individual which occurs by reason of:

(i) such individual’s involuntary dismissal or discharge by the Corporation for reasons other than Misconduct, or

(ii) such individual’s voluntary resignation following (A) a change in his or her position with the Corporation which materially reduces his or her duties and responsibilities or the level of management to which he or she reports, (B) a reduction in his or her level of compensation (including base salary, fringe benefits and target bonus under any corporate-performance based bonus or incentive programs) by more than fifteen percent (15%) or (C) a relocation of such individual’s place of employment by more than fifty (50) miles, provided and only if such change, reduction or relocation is effected without the individual’s consent.

N. Misconduct shall mean the commission of any act of fraud, embezzlement or dishonesty by the Optionee or Participant, any unauthorized use or disclosure by such person of confidential information or trade secrets of the Corporation (or any Parent or Subsidiary), or any other intentional misconduct by such person adversely affecting the business or affairs of the Corporation (or any Parent or Subsidiary) in a material manner. The foregoing definition shall not in any way preclude or restrict the right of the Corporation (or any Parent or Subsidiary) to discharge or dismiss any Optionee, Participant or other person in the Service of the Corporation (or any Parent or Subsidiary) for any other acts or omissions, but such other acts or omissions shall not be deemed, for purposes of the Plan, to constitute grounds for termination for Misconduct.

O. 1933 Act shall mean the Securities Act of 1933, as amended.

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P. 1934 Act shall mean the Securities Exchange Act of 1934, as amended.

Q. Non-Statutory Option shall mean an option not intended to satisfy the requirements of Code Section 422.

R. Option Grant Program shall mean the option grant program in effect under the Plan.

S. Optionee shall mean any person to whom an option is granted under the Plan.

T. Parent shall mean any corporation (other than the Corporation) in an unbroken chain of corporations ending with the Corporation, provided each corporation in the unbroken chain (other than the Corporation) owns, at the time of the determination, stock possessing fifty percent (50%) or more of the total combined voting power of all classes of stock in one of the other corporations in such chain.

U. Participant shall mean any person who is issued shares of Common Stock under the Stock Issuance Program or to whom restricted stock units or share rights are awarded under such program.

V. Plan shall mean the Corporation’s 2012 Stock Option/Stock Issuance Plan, as set forth in this document.

W. Plan Administrator shall mean either the Board or the Committee acting in its capacity as administrator of the Plan.

X. Service shall mean the performance of services for the Corporation (or any Parent or Subsidiary, whether now existing or subsequently established) by a person in the capacity of an Employee, a non-employee member of the board of directors or a consultant or independent advisor, except to the extent otherwise specifically provided in the documents evidencing the option grant or stock issuance. For purposes of the Plan, an Optionee or Participant shall be deemed to cease Service immediately upon the occurrence of the either of the following events: (i) the Optionee or Participant no longer performs services in any of the foregoing capacities for the Corporation or any Parent or Subsidiary or (ii) the entity for which the Optionee or Participant is performing such services ceases to remain a Parent or Subsidiary of the Corporation, even though the Optionee or Participant may subsequently continue to perform services for that entity. Service shall not be deemed to cease during a period of military leave, sick leave or other personal leave approved by the Corporation; provided, however, that for a leave which exceeds three (3) months, Service shall be deemed, for purposes of determining the period within which any outstanding option held by the Optionee in question may be exercised as an Incentive Option, to cease on the first day immediately following the expiration of such three (3)-month period, unless that Optionee is provided with the right to return to Service following such leave either by statute or by written contract. Except to the extent otherwise required by law or expressly authorized by the Plan Administrator or by the Corporation’s written policy on leaves of absence, no Service credit shall be given for vesting purposes for any period the Optionee or Participant is on a leave of absence.

Y. Stock Exchange shall mean the American Stock Exchange, the Nasdaq Global or Global Select Market or the New York Stock Exchange.

Z. Stock Issuance Agreement shall mean the agreement entered into by the Corporation and the Participant at the time of issuance of shares of Common Stock under the Stock Issuance Program.

AA. Stock Issuance Program shall mean the stock issuance program in effect under the Plan.

BB. Subsidiary shall mean any corporation (other than the Corporation) in an unbroken chain of corporations beginning with the Corporation, provided each corporation (other than the last corporation) in the unbroken chain owns, at the time of the determination, stock possessing fifty percent (50%) or more of the total combined voting power of all classes of stock in one of the other corporations in such chain.

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CC. 10% Stockholder shall mean the owner of stock (as determined under Code Section 424(d)) possessing more than ten percent (10%) of the total combined voting power of all classes of stock of the Corporation (or any Parent or Subsidiary).

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