Exhibit 10.3

LOCK-UP AGREEMENT

Korro Bio, Inc. One Kendall Square Building 600-700, Suite 6-401 Cambridge, MA 02139 [                 ], 2023

Ladies and Gentlemen:

The undersigned signatory of this lock-up agreement (this “Lock-Up Agreement”) understands that Frequency Therapeutics, Inc., a Delaware corporation (“Frequency”), has entered into an Agreement and Plan of Merger, dated as of July 14, 2023 (as the same may be amended from time to time, the “Merger Agreement”) with Frequency Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of Frequency, and Korro Bio, Inc., a Delaware corporation (the “Company”). Capitalized terms used but not otherwise defined herein shall have the respective meanings ascribed to such terms in the Merger Agreement.

As a condition and inducement to each of the parties to enter into the Merger Agreement, and for other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the undersigned hereby irrevocably agrees that, subject to the exceptions set forth herein, without the prior written consent of Frequency and, solely prior to the Closing, the Company, the undersigned will not, during the period commencing upon the Closing and ending on the date that is 180 days after the Closing Date (the “Restricted Period”):

(i) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, or otherwise transfer or dispose of, directly or indirectly, any shares of Frequency Common Stock or any securities convertible into or exercisable or exchangeable for Frequency Common Stock (including without limitation, Frequency Common Stock or such other securities which may be deemed to be beneficially owned (as such term is used in Rule 13d-3 of the Exchange Act) by the undersigned in accordance with the rules and regulations of the SEC and securities of Frequency which may be issued upon exercise of an option to purchase Frequency Common Stock or warrant or settlement of a Frequency Restricted Stock Unit) that are currently or hereafter owned by the undersigned (collectively, the “Undersigned’s Shares”), or publicly disclose the intention to make any such offer, sale, pledge, grant, transfer or disposition;

(ii) enter into any swap, short sale, hedge or other agreement that transfers, in whole or in part, any of the economic consequences of ownership of the Undersigned’s Shares regardless of whether any such transaction described in clause (i) above or this clause (ii) is to be settled by delivery of Frequency Common Stock or other securities, in cash or otherwise; or

(iii) make any demand for, or exercise any right with respect to, the registration of any shares of Frequency Common Stock or any security convertible into or exercisable or exchangeable for Frequency Common Stock (other than such rights set forth in the Merger Agreement or, to the extent the undersigned is party to it, the Registration Rights Agreement dated as of [ ], 2023 (as the same may be amended from time to time) between the Company and the other parties thereto)).

The restrictions and obligations contemplated by this Lock-Up Agreement shall not apply to:

(a) transfers of the undersigned’s Shares:

(i) if the undersigned is a natural person, (A) to any person related to the undersigned by blood or adoption who is an immediate family member of the undersigned, or by marriage or domestic partnership (a “Family Member”), or to a trust formed for the direct or indirect benefit of the undersigned or any of the undersigned’s Family Members, (B) to the undersigned’s estate, following the death of the undersigned, by will, intestacy or other operation of Law, (C) as a bona fide gift or a charitable contribution, as such term is described in Section 501(c)(3) of the Internal Revenue Code of 1986, as amended, (D) by operation of Law pursuant to a qualified domestic order or in connection with a divorce settlement, or (E) to any partnership, corporation or limited liability company which is controlled by the undersigned and/or by any such Family Member(s);

(ii) if the undersigned is a corporation, partnership, limited liability company or other entity, (A) to another corporation, partnership, limited liability company, or other entity that is an affiliate (as defined under Rule 12b-2 of the Exchange Act) of the undersigned, including investment funds or other entities under common control or management or advisement with the undersigned (including, for the avoidance of doubt, where the undersigned is a partnership, to its general partner or a successor partnership or fund, or any other funds managed by such partnership), (B) as a distribution or dividend to equity holders, (including, without limitation, current or former general or limited partners, members or managers (or to the estates of any of the foregoing), as applicable, of the undersigned (including upon the liquidation and dissolution of the undersigned pursuant to a plan of liquidation approved by the undersigned’s equity holders), (C) as a bona fide gift or a charitable contribution, as such term is described in Section 501(c)(3) of the Internal Revenue Code of 1986, as amended, (D) transfers or dispositions not involving a change in beneficial ownership or (E) with prior written consent of Frequency; or

(iii) if the undersigned is a trust, to any grantors or beneficiaries of the trust;

provided that, in the case of any transfer or distribution pursuant to this clause (a), such transfer is not for value and each donee, heir, beneficiary or other transferee or distributee shall sign and deliver to Frequency a lock-up agreement in the form of this Lock-Up Agreement with respect to the shares of Frequency Common Stock or such other securities that have been so transferred or distributed;

(b) the exercise of an option to purchase Frequency Common Stock (including a net or cashless exercise of an option to purchase Frequency Common Stock), and any related transfer of shares of Frequency Common Stock to Frequency or sale of Frequency Common Stock in the open market, in each case, for the purpose of paying the exercise price of such options or for paying taxes (including estimated taxes) during the Restricted Period due as a result of the exercise of such options; provided that, for the avoidance of doubt, the underlying shares of Frequency Common Stock held by the undersigned following such exercise and any such open market sales shall continue to be subject to the restrictions on transfer set forth in this Lock-Up Agreement;

(c) the disposition (including a forfeiture or repurchase) to Frequency of any shares of restricted stock granted pursuant to the terms of any employee benefit plan or restricted stock purchase agreement;

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(d) transfers to Frequency, or sales of Frequency Common Stock in the open market, in connection with the vesting of any restricted stock unit or settlement of any other equity award that represents the right to receive shares of Frequency Common Stock settled in Frequency Common Stock, in each case, to pay any tax withholding obligations due during the Restricted Period; provided that, for the avoidance of doubt, the underlying shares of Frequency Common Stock held by the undersigned following such exercise and any such open market sales shall continue to be subject to the restrictions on transfer set forth in this Lock-Up Agreement;

(f) the establishment of a trading plan pursuant to Rule 10b5-1 under the Exchange Act (a “10b5-1 Plan”) for the transfer of Frequency Common Stock; provided that such plan does not provide for any transfers of Frequency Common Stock during the Restricted Period, or the sale of Frequency Common Stock pursuant to a 10b5-1 Plan existing as of the date of the Merger Agreement (which, for clarity, shall not be amended during the Restricted Period, but may be terminated during the Restricted Period);

(f) transfers, sales, dispositions, or the entering into of transactions (including, without limitation, any swap, hedge or similar agreement) by the undersigned of or relating to shares of capital stock or other securities of Frequency purchased or acquired by the undersigned on the open market, in a public offering by Frequency, or that otherwise do not involve or relate to shares of Frequency Common Stock issued pursuant to the Merger Agreement in respect of shares of the Company;

(g) pursuant to a bona-fide third party tender offer, merger, consolidation or other similar transaction made to all holders of Frequency’s capital stock involving a change of control of Frequency, provided that in the event that such tender offer, merger, consolidation or other such transaction is not completed, the Undersigned’s Shares shall remain subject to the restrictions contained in this Lock-Up Agreement;

(h) pursuant to an order of a court or regulatory agency; or

(i) transfers, sales, dispositions, or the entering into of transaction (including, without limitation, any swap, hedge, or similar agreement), by the undersigned relating to shares of Frequency Common Stock issued pursuant to the Merger Agreement in respect of shares of the Company, if any, purchased from the Company pursuant to the Concurrent Financing (as defined in the Merger Agreement) or issued in exchange for, or on conversion or exercise of, any securities issued as part of the Concurrent Financing.

And provided, further, that, with respect to each of (a), (b), (c), (d) and (e) above, no filing by any party (including any donor, donee, transferor, transferee, distributor or distributee) under Section 16 of the Exchange Act or other public announcement shall be made voluntarily in connection with such transfer or disposition during the Restricted Period; provided that (i) any filing under Section 16 of the Exchange Act made during the Restricted Period shall clearly indicate in the footnotes thereto that such filing relates to the circumstances described in (a), (b), (c), (d) or (e), as applicable and (ii) the foregoing shall not prevent the undersigned from filing a Form 13F, Schedule 13G or Schedule 13D, or any amendment thereto, or from disclosing its holdings in Frequency as required by law or regulation or its internal disclosure policies in the ordinary course of business.

Any attempted transfer in violation of this Lock-Up Agreement will be of no effect and null and void, regardless of whether the purported transferee has any actual or constructive knowledge of the transfer restrictions set forth in this Lock-Up Agreement, and will not be recorded on the share register of Frequency. In furtherance of the foregoing, the undersigned agrees that Frequency and any duly appointed

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transfer agent for the registration or transfer of the securities described herein are hereby authorized to decline to make any transfer of securities if such transfer would constitute a violation or breach of this Lock-Up Agreement. Frequency may cause the legend set forth below, or a legend substantially equivalent thereto, to be placed upon any certificate(s) or other documents, ledgers or instruments evidencing the undersigned’s ownership of Frequency Common Stock:

THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO AND MAY ONLY BE TRANSFERRED IN COMPLIANCE WITH A LOCK-UP AGREEMENT, A COPY OF WHICH IS ON FILE AT THE PRINCIPAL OFFICE OF THE COMPANY.

The undersigned hereby represents and warrants that the undersigned has full power and authority to enter into this Lock-Up Agreement. All authority herein conferred or agreed to be conferred and any obligations of the undersigned shall be binding upon the successors, assigns, heirs or personal representatives of the undersigned.

The undersigned understands that if the Merger Agreement is terminated for any reason, the undersigned shall be released from all obligations under this Lock-Up Agreement. The undersigned understands that Frequency and the Company are proceeding with the Contemplated Transactions in reliance upon this Lock-Up Agreement. Notwithstanding anything to the contrary contained herein, this letter agreement will automatically terminate and the undersigned shall be released from all obligations under this letter agreement upon the earliest to occur, if any, of (i) the Company advising the undersigned in writing that it has determined not to proceed with the Contemplated Transactions or (ii) the Merger Agreement being terminated.

Any and all remedies herein expressly conferred upon Frequency or the Company will be deemed cumulative with and not exclusive of any other remedy conferred hereby, or by Law or equity, and the exercise by Frequency or the Company of any one remedy will not preclude the exercise of any other remedy. The undersigned agrees that irreparable damage could occur to Frequency and/or the Company in the event that any provision of this Lock-Up Agreement were not performed in accordance with its specific terms or were otherwise breached. It is accordingly agreed that Frequency and the Company shall be entitled to seek an injunction or injunctions to prevent breaches of this Lock-Up Agreement and to enforce specifically the terms and provisions hereof in any court of the United States or any state having jurisdiction, this being in addition to any other remedy to which Frequency or the Company is entitled at Law or in equity, and the undersigned waives any bond, surety or other security that might be required of Frequency or the Company with respect thereto.

In the event that any holder of Frequency’s securities that are subject to a substantially similar agreement entered into by such holder, other than the undersigned, is permitted by Frequency (or prior to the Closing, the Company), including through any written consent granted under subparagraph a(ii)(E) above, to sell or otherwise transfer or dispose of shares of Frequency Common Stock for value other than as permitted by this or a substantially similar agreement entered into by such holder or is granted an early release from the restrictions described herein during the Restricted Period, the same percentage of shares of the Undersigned’s Shares shall be immediately and fully released from any remaining restrictions set forth herein (the “Pro-Rata Release”); provided, however, that such Pro-Rata Release shall not be applied unless and until permission or early release has been granted by Frequency, and solely prior to the Closing, the Company, to an equity holder or equity holders to sell or otherwise transfer or dispose of all or a portion of such equity holder’s shares of Frequency Common Stock that, when combined with all such other such permissions and early releases, represent an aggregate amount in excess of 1% of the number of shares of Frequency Common Stock originally subject to a substantially similar agreements. Frequency shall notify the undersigned of any Pro Rata Release of its shares on the same day that any permission that triggers the Pro Rata Release is granted.

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Upon the release of any of the Undersigned’s Shares from this Lock-Up Agreement, Frequency will cooperate with the undersigned to facilitate the timely preparation and delivery of certificates representing the Undersigned’s Shares without the restrictive legend above or the withdrawal of any stop transfer instructions.

This Lock-Up Agreement and any claim, controversy or dispute arising under or related to this Lock-Up Agreement shall be governed by and construed in accordance with the Laws of the State of Delaware, without regard to the conflict of Laws principles thereof.

This Lock-Up Agreement may be executed in several counterparts, each of which shall be deemed an original and all of which shall constitute one and the same instrument. The exchange of a fully executed Lock-Up Agreement (in counterparts or otherwise) by Frequency, the Company and the undersigned by facsimile, electronic mail (including any electronic signature covered by the U.S. federal ESIGN Act of 2000, Uniform Electronic Transactions Act, the Electronic Signatures and Records Act or other applicable law, e.g., www.docusign.com) or electronic transmission in .pdf format shall be sufficient to bind such parties to the terms and conditions of this Lock-Up Agreement.

(Signature Page Follows)

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	Very truly yours,
Print Name of Stockholder:	[                                     ]
	Signature (for individuals):

Signature (for entities):
By:
	Name:
	Title:

Accepted and Agreed By Frequency Therapeutics, Inc.:
By:
	Name:
	Title:
Accepted and Agreed by Korro Bio, Inc.:
By:
	Name:
	Title:

[Signature Page to Lock-up Agreement]