Document:

EX-10.3

 Exhibit 10.3 

TSCAN THERAPEUTICS, INC. 

2021 EQUITY INCENTIVE PLAN 

(As Adopted on April 22, 2021) 

 TSCAN THERAPEUTICS, INC. 

2021 EQUITY INCENTIVE PLAN 

ARTICLE 1.    INTRODUCTION. 

The Board adopted the Plan to become effective immediately, although no Awards may be granted prior to the IPO Date. The purpose of the Plan is
to promote the long-term success of the Company and the creation of stockholder value by (a) encouraging Service Providers to focus on critical long-range corporate objectives, (b) encouraging the attraction and retention of Service
Providers with exceptional qualifications and (c) linking Service Providers directly to stockholder interests through increased stock ownership. The Plan seeks to achieve this purpose by providing for Awards in the form of Options (which may be
ISOs or NSOs), SARs, Restricted Shares and Restricted Stock Units. Capitalized terms used in this Plan are defined in Article 14. 
 ARTICLE
2.    ADMINISTRATION. 
 2.1    General. The Plan may be administered by the
Board or one or more Committees to which the Board (or an authorized Board committee) has delegated authority. If administration is delegated to a Committee, the Committee shall have the powers theretofore possessed by the Board, including, to the
extent permitted by applicable law, the power to delegate to a subcommittee any of the administrative powers the Committee is authorized to exercise (and references in this Plan to either the Board or the Administrator shall hereafter also encompass
the Committee or subcommittee, as applicable). The Board may abolish the Committee’s delegation at any time and the Board shall at all times also retain the authority it has delegated to the Committee. The Administrator shall comply with rules
and regulations applicable to it, including under the rules of any exchange on which the Common Shares are traded, and shall have the authority and be responsible for such functions as have been assigned to it. 

2.2    Section 16. To the extent desirable to qualify transactions hereunder as exempt under Exchange Act
Rule 16b-3, the transactions contemplated hereunder will be approved by the entire Board or a Committee of two or more “non-employee directors” within the
meaning of Exchange Act Rule 16b-3. 
 2.3    Powers of
Administrator. Subject to the terms of the Plan, and in the case of a Committee, subject to the specific duties delegated to the Committee, the Administrator shall have the authority to (a) select the Service Providers who are to receive
Awards under the Plan, (b) determine the type, number, vesting requirements and other features and conditions of such Awards, (c) interpret the Plan and Awards granted under the Plan, (d) determine whether, when and to what extent an
Award has become vested and/or exercisable and whether any performance-based vesting conditions have been satisfied, (e) make, amend and rescind rules relating to the Plan and Awards granted under the Plan, including rules relating to sub-plans established for the purposes of satisfying applicable foreign laws or for qualifying for favorable tax treatment under applicable foreign laws, (f) impose such restrictions, conditions or limitations
as it determines appropriate as to the timing and manner of any resales by a Participant of any Common Shares issued pursuant to an Award, including restrictions under an insider trading policy and restrictions as to the use of a specified brokerage
firm for such resales, 

 
and (g) make all other decisions relating to the operation of the Plan and Awards granted under the Plan. In addition, with regard to the terms and conditions of Awards granted to Service
Providers outside of the United States, the Administrator may vary from the provisions of the Plan (other than any requiring stockholder approval pursuant to Section 13.3) to the extent it determines it necessary and appropriate to do so. 

2.4    Effect of Administrator’s Decisions. The Administrator’s decisions, determinations and
interpretations shall be final and binding on all interested parties. 
 2.5    Governing Law. The Plan
shall be governed by, and construed in accordance with, the laws of the State of Delaware (except its choice-of-law provisions). 

ARTICLE 3.    SHARES AVAILABLE FOR GRANTS. 

3.1    Basic Limitation. Common Shares issued pursuant to the Plan may be authorized but unissued shares or
treasury shares. The aggregate number of Common Shares issued under the Plan shall not exceed the sum of (a) 26,880,000 Common Shares, (b) any Common Shares subject to awards granted under the Predecessor Plan that are outstanding on the
IPO Date that subsequently are forfeited, expire or lapse unexercised or unsettled and Common Shares issued pursuant to awards granted under the Predecessor Plan that are outstanding on the IPO Date and that are subsequently forfeited to or
reacquired by the Company, (c) the number of Common Shares reserved under the Predecessor Plan that are not issued or subject to outstanding awards under the Predecessor Plan on the IPO Date and (d) the additional Common Shares described
in Articles 3.2 and 3.3. The Company shall reserve and keep available such number of Common Shares as will be sufficient to satisfy the requirements of the Plan. The numerical limitations in this Article 3.1 shall be subject to
adjustment pursuant to Article 9. 
 3.2    Annual Increase in Shares. On the first day of each fiscal year of the Company during the term of the Plan, commencing in 2022 and ending in (and including) 2031, the aggregate number of Common Shares that may be issued under the Plan shall
automatically increase by a number equal to the lesser of (a) 5% of the total number of Common Shares actually issued and outstanding on the last day of the preceding fiscal year or (b) a number of Common Shares determined by the Board.

 3.3    Shares Returned to Reserve. To the extent that Options, SARs, Restricted Stock Units or other
Awards are forfeited, cancelled or expire for any reason before being exercised or settled in full, the Common Shares subject to such Awards shall again become available for issuance under the Plan. If SARs are exercised or Restricted Stock Units
are settled, then only the number of Common Shares (if any) actually issued to the Participant upon exercise of such SARs or settlement of such Restricted Stock Units, as applicable, shall reduce the number of Common Shares available under
Article 3.1 and the balance shall again become available for issuance under the Plan. If Restricted Shares or Common Shares issued upon the exercise of Options are reacquired by the Company pursuant to a forfeiture provision, repurchase right
or for any other reason, then such Common Shares shall again become available for issuance under the Plan. Common Shares applied to pay the Exercise Price of Options or to satisfy tax withholding obligations related to any Award shall again become
available for issuance under the Plan. To the extent that an Award is settled in cash rather than Common Shares, the cash settlement shall not reduce the number of Shares available for issuance under the Plan. 

 3.4    Awards Not Reducing Share Reserve. To the extent
permitted under applicable exchange listing standards, any dividend equivalents paid or credited under the Plan with respect to Restricted Stock Units shall not be applied against the number of Common Shares that may be issued under the Plan,
whether or not such dividend equivalents are converted into Restricted Stock Units. In addition, Common Shares subject to Substitute Awards granted by the Company shall not reduce the number of Common Shares that may be issued under Article 3.1, nor
shall shares subject to Substitute Awards again be available for Awards under the Plan in the event of any forfeiture, expiration or cash settlement of such Substitute Awards. 

3.5    Code Section 422 and Other Limits. Subject to adjustment in accordance with
Article 9: 
 (a)    No more than 26,880,000 Common Shares may be issued under the Plan upon the exercise of ISOs. 

(b)    The aggregate grant date fair value of Awards granted to an Outside Director during any one fiscal year of the
Company, together with the value of any cash compensation paid to the Outside Director during such fiscal year, may not exceed $750,000 (on a per-Director basis); provided however that the limitation that will
apply in the fiscal year in which the Outside Director is initially appointed or elected to the Board shall instead be $1,500,000. For purposes of this limitation, the grant date fair value of an Award shall be determined in accordance with the
assumptions that the Company uses to estimate the value of share-based payments for financial reporting purposes. For the sake of clarity, neither Awards granted, nor compensation paid, to an individual for his or her service as an Employee or
Consultant, but not as an Outside Director, shall count towards this limitation. 
 ARTICLE 4.    ELIGIBILITY. 

4.1    Incentive Stock Options. Only Employees who are common-law
employees of the Company, a Parent or a Subsidiary shall be eligible for the grant of ISOs. In addition, an Employee who owns more than 10% of the total combined voting power of all classes of outstanding stock of the Company or any of its Parents
or Subsidiaries shall not be eligible for the grant of an ISO unless the additional requirements set forth in Code Section 422(c)(5) are satisfied. 

4.2    Other Awards. Awards other than ISOs may be granted to both Employees and other Service
Providers. 
 ARTICLE 5.    OPTIONS. 

5.1    Stock Option Agreement. Each grant of an Option under the Plan shall be evidenced by a Stock Option
Agreement between the Optionee and the Company. Such Option shall be subject to all applicable terms of the Plan and may be subject to any other terms that are not inconsistent with the Plan. The Stock Option Agreement shall specify whether the
Option is intended to be an ISO or an NSO. The provisions of the various Stock Option Agreements entered into under the Plan need not be identical. 

5.2    Number of Shares. Each Stock Option Agreement shall specify the number of Common Shares subject to
the Option, which number shall adjust in accordance with Article 9. 

 5.3    Exercise Price. Each Stock Option Agreement shall
specify the Exercise Price, which shall not be less than 100% of the Fair Market Value of a Common Share on the date of grant. The preceding sentence shall not apply to an Option that is a Substitute Award granted in a manner that would satisfy the
requirements of Code Section 409A and, if applicable, Code Section 424(a). 
 5.4    Exercisability
and Term. Each Stock Option Agreement shall specify the date or event when all or any installment of the Option is to become vested and/or exercisable. The vesting and exercisability conditions applicable to the Option may include service-based
conditions, performance-based conditions, such other conditions as the Administrator may determine, or any combination of such conditions. The Stock Option Agreement shall also specify the term of the Option; provided that, except to the extent
necessary to comply with applicable foreign law, the term of an Option shall in no event exceed 10 years from the date of grant. A Stock Option Agreement may provide for accelerated vesting and/or exercisability upon certain specified events
and may provide for expiration prior to the end of its term in the event of the termination of the Optionee’s service. 

5.5    Death of Optionee. After an Optionee’s death, any vested and exercisable Options held by
such Optionee may be exercised by his or her beneficiary or beneficiaries. Each Optionee may designate one or more beneficiaries for this purpose by filing the prescribed form with the Company. A beneficiary designation may be changed by filing the
prescribed form with the Company at any time before the Optionee’s death. If no beneficiary was designated or if no designated beneficiary survives the Optionee, then any vested and exercisable Options held by the Optionee may be exercised by
his or her estate. 
 5.6    Modification or Assumption of Options. Within the limitations of the Plan,
the Administrator may modify, extend or assume outstanding options. The foregoing notwithstanding, no modification of an Option shall, without the consent of the Optionee, materially impair his or her rights or obligations under such Option.
Notwithstanding anything in this Plan to the contrary, and except for the adjustments provided in Article 9, neither the Administrator nor any other person may (a) decrease the exercise price for any outstanding Option after the date of grant,
(b) cancel or allow an Optionee to surrender an outstanding Option to the Company in exchange for cash or as consideration for the grant of a new Option with a lower exercise price or the grant of another type of Award the effect of which is to
reduce the exercise price of any outstanding Option, or (c) take any other action with respect to an Option that would be treated as a repricing under the rules and regulations of the Nasdaq Stock Market (or such other principal U.S. national
securities exchange on which the Common Shares are traded). 
 5.7    Buyout Provisions. Except to the
extent prohibited by Article 5.6, the Administrator may at any time (a) offer to buy out for a payment in cash or cash equivalents an Option previously granted or (b) authorize an Optionee to elect to cash out an Option previously granted,
in either case at such time and based upon such terms and conditions as the Administrator shall establish. 

5.8    Payment for Option Shares. The entire Exercise Price of Common Shares issued upon exercise of Options
shall be payable in cash or cash equivalents at the time when such Common Shares are purchased. In addition, the Administrator may, in its sole discretion 

 
and to the extent permitted by applicable law, accept payment of all or a portion of the Exercise Price through any one or a combination of the following forms or methods: 

(a)    Subject to any conditions or limitations established by the Administrator, by surrendering, or attesting to the
ownership of, Common Shares that are already owned by the Optionee with a value on the date of surrender equal to the aggregate exercise price of the Common Shares as to which such Option will be exercised; 

(b)    By delivering (on a form prescribed by the Company) an irrevocable direction to a securities broker approved by the
Company to sell all or part of the Common Shares being purchased under the Plan and to deliver all or part of the sales proceeds to the Company; 

(c)    Subject to such conditions and requirements as the Administrator may impose from time to time, through a net
exercise procedure; or 
 (d)    Through any other form or method consistent with applicable laws, regulations and
rules. 
 ARTICLE 6.    STOCK APPRECIATION RIGHTS. 

6.1    SAR Agreement. Each grant of a SAR under the Plan shall be evidenced by a SAR Agreement between the
Optionee and the Company. Such SAR shall be subject to all applicable terms of the Plan and may be subject to any other terms that are not inconsistent with the Plan. The provisions of the various SAR Agreements entered into under the Plan need not
be identical. 
 6.2    Number of Shares. Each SAR Agreement shall specify the number of Common Shares to
which the SAR pertains, which number shall adjust in accordance with Article 9. 
 6.3    Exercise
Price. Each SAR Agreement shall specify the Exercise Price, which shall in no event be less than 100% of the Fair Market Value of a Common Share on the date of grant. The preceding sentence shall not apply to a SAR that is a Substitute Award
granted in a manner that would satisfy the requirements of Code Section 409A. 
 6.4    Exercisability
and Term. Each SAR Agreement shall specify the date when all or any installment of the SAR is to become vested and exercisable. The vesting and exercisability conditions applicable to the SAR may include service-based conditions,
performance-based conditions, such other conditions as the Administrator may determine, or any combination thereof. The SAR Agreement shall also specify the term of the SAR; provided that except to the extent necessary to comply with applicable
foreign law, the term of a SAR shall not exceed 10 years from the date of grant. A SAR Agreement may provide for accelerated vesting and exercisability upon certain specified events and may provide for expiration prior to the end of its term in the
event of the termination of the Optionee’s service. 
 6.5    Exercise of SARs. Upon exercise of a
SAR, the Optionee (or any person having the right to exercise the SAR after his or her death) shall receive from the Company (a) Common Shares, (b) cash or (c) a combination of Common Shares and cash, as the Administrator shall
determine. The amount of cash and/or the Fair Market Value of Common Shares received upon exercise of SARs shall, in the aggregate, not exceed the amount by which the Fair Market Value 

 
(on the date of surrender) of the Common Shares subject to the SARs exceeds the Exercise Price. If, on the date when a SAR expires, the Exercise Price is less than the Fair Market Value on such
date but any portion of such SAR has not been exercised or surrendered, then such SAR shall automatically be deemed to be exercised as of such date with respect to such portion. A SAR Agreement may also provide for an automatic exercise of the SAR
on an earlier date. 
 6.6    Death of Optionee. After an Optionee’s death, any vested and
exercisable SARs held by such Optionee may be exercised by his or her beneficiary or beneficiaries. Each Optionee may designate one or more beneficiaries for this purpose by filing the prescribed form with the Company. A beneficiary designation may
be changed by filing the prescribed form with the Company at any time before the Optionee’s death. If no beneficiary was designated or if no designated beneficiary survives the Optionee, then any vested and exercisable SARs held by the Optionee
at the time of his or her death may be exercised by his or her estate. 
 6.7    Modification or Assumption of
SARs. Within the limitations of the Plan, the Administrator may modify, extend or assume outstanding SARs. The foregoing notwithstanding, no modification of a SAR shall, without the consent of the Optionee, materially impair his or her rights or
obligations under such SAR. Notwithstanding anything in this Plan to the contrary, and except for the adjustments provided in Article 9, neither the Administrator nor any other person may (a) decrease the exercise price for any outstanding SAR
after the date of grant, (b) cancel or allow an Optionee to surrender an outstanding SAR to the Company in exchange for cash or as consideration for the grant of a new SAR with a lower exercise price or the grant of another type of Award the
effect of which is to reduce the exercise price of any outstanding SAR, or (c) take any other action with respect to a SAR that would be treated as a repricing under the rules and regulations of the Nasdaq Stock Market (or such other principal
U.S. national securities exchange on which the Common Shares are traded). 
 ARTICLE 7.    RESTRICTED SHARES. 

7.1    Restricted Stock Agreement. Each grant of Restricted Shares under the Plan shall be evidenced by a
Restricted Stock Agreement between the recipient and the Company. Such Restricted Shares shall be subject to all applicable terms of the Plan and may be subject to any other terms that are not inconsistent with the Plan. The provisions of the
various Restricted Stock Agreements entered into under the Plan need not be identical. 
 7.2    Payment for
Awards. Restricted Shares may be sold or awarded under the Plan for such consideration as the Administrator may determine, including (without limitation) cash, cash equivalents, property, cancellation of other equity awards, promissory notes,
past services and future services, and such other methods of payment as are permitted by applicable law. 

7.3    Vesting Conditions. Each Award of Restricted Shares may or may not be subject to vesting and/or other
conditions as the Administrator may determine. Vesting shall occur, in full or in installments, upon satisfaction of the conditions specified in the Restricted Stock Agreement. A Restricted Stock Agreement may provide for accelerated vesting upon
certain specified events. 
 7.4    Voting and Dividend Rights. The holders of Restricted Shares awarded
under the Plan shall have the same voting, dividend and other rights as the Company’s other stockholders, unless the Administrator otherwise provides. A Restricted Stock Agreement, 

 
however, may require that any cash dividends paid on Restricted Shares (a) be accumulated and paid when such Restricted Shares vest, or (b) be invested in additional Restricted Shares.
Such additional Restricted Shares shall be subject to the same conditions and restrictions as the shares subject to the Award with respect to which the dividends were paid. In addition, if any dividends or other distributions are paid in Common
Shares, such Common Shares shall be subject to the same restrictions on transferability and forfeitability as the Restricted Shares with respect to which they were paid. 

7.5    Modification or Assumption of Restricted Shares. Within the limitations of the Plan, the
Administrator may modify or assume outstanding Restricted Shares or may accept the cancellation of outstanding restricted shares (whether granted by the Company or by another issuer) in return for the grant of new Restricted Shares for the same or a
different number of shares or in return for the grant of a different type of Award. The foregoing notwithstanding, no modification of Restricted Shares shall, without the consent of the Participant, materially impair his or her rights or obligations
under such Restricted Shares. 
 ARTICLE 8.    RESTRICTED STOCK UNITS. 

8.1    Restricted Stock Unit Agreement. Each grant of Restricted Stock Units under the Plan shall be
evidenced by a Restricted Stock Unit Agreement between the recipient and the Company. Such Restricted Stock Units shall be subject to all applicable terms of the Plan and may be subject to any other terms that are not inconsistent with the Plan. The
provisions of the various Restricted Stock Unit Agreements entered into under the Plan need not be identical. 

8.2    Payment for Awards. To the extent that an Award is granted in the form of Restricted Stock Units, no
cash consideration shall be required of the Award recipients. 
 8.3    Vesting Conditions. Each Award of
Restricted Stock Units may or may not be subject to vesting, as determined by the Administrator. Vesting shall occur, in full or in installments, upon satisfaction of the conditions specified in the Restricted Stock Unit Agreement. Vesting
conditions may include service-based conditions, performance-based conditions, such other conditions as the Administrator may determine, or any combination thereof. A Restricted Stock Unit Agreement may provide for accelerated vesting upon certain
specified events. 
 8.4    Voting and Dividend Rights. The holders of Restricted Stock Units shall have
no voting rights. Prior to settlement or forfeiture, Restricted Stock Units awarded under the Plan may, at the Administrator’s discretion, provide for a right to dividend equivalents. Such right entitles the holder to be credited with an amount
equal to all cash dividends paid on one Common Share while the Restricted Stock Unit is outstanding. Dividend equivalents may be converted into additional Restricted Stock Units. Settlement of dividend equivalents may be made in the form of cash, in
the form of Common Shares, or in a combination of both. Prior to distribution, any dividend equivalents shall be subject to the same conditions and restrictions as the Restricted Stock Units to which they attach. 

8.5    Form and Time of Settlement of Restricted Stock Units. Settlement of vested Restricted
Stock Units may be made in the form of (a) cash, (b) Common Shares or (c) any combination of both, as determined by the Administrator. The actual number of Restricted Stock Units eligible for settlement may be larger or smaller than
the number included in the original 

 
Award, based on predetermined performance factors. Methods of converting Restricted Stock Units into cash may include (without limitation) a method based on the average value of Common Shares
over a series of trading days. Vested Restricted Stock Units shall be settled in such manner and at such time(s) as specified in the Restricted Stock Unit Agreement. Until an Award of Restricted Stock Units is settled, the number of such Restricted
Stock Units shall be subject to adjustment pursuant to Article 9. 
 8.6    Death of Recipient. Any
Restricted Stock Units that become payable after the recipient’s death shall be distributed to the recipient’s beneficiary or beneficiaries. Each recipient of Restricted Stock Units under the Plan may designate one or more beneficiaries
for this purpose by filing the prescribed form with the Company. A beneficiary designation may be changed by filing the prescribed form with the Company at any time before the Award recipient’s death. If no beneficiary was designated or if no
designated beneficiary survives the Award recipient, then any Restricted Stock Units that become payable after the recipient’s death shall be distributed to the recipient’s estate. 

8.7    Modification or Assumption of Restricted Stock Units. Within the limitations of the
Plan, the Administrator may modify or assume outstanding restricted stock units or may accept the cancellation of outstanding restricted stock units (whether granted by the Company or by another issuer) in return for the grant of new Restricted
Stock Units for the same or a different number of shares or in return for the grant of a different type of Award. The foregoing notwithstanding, no modification of a Restricted Stock Unit shall, without the consent of the Participant, materially
impair his or her rights or obligations under such Restricted Stock Unit. 

8.8    Creditors’ Rights. A holder of Restricted Stock Units shall have no rights other
than those of a general creditor of the Company. Restricted Stock Units represent an unfunded and unsecured obligation of the Company, subject to the terms and conditions of the applicable Restricted Stock Unit Agreement. 

ARTICLE 9.    ADJUSTMENTS; DISSOLUTIONS AND LIQUIDATIONS; CORPORATE TRANSACTIONS. 

9.1    Adjustments. In the event of a subdivision of the outstanding Common Shares, a declaration of a
dividend payable in Common Shares, a combination or consolidation of the outstanding Common Shares (by reclassification or otherwise) into a lesser number of Common Shares or any other increase or decrease in the number of issued Common Shares
effected without receipt of consideration by the Company, proportionate adjustments shall be made to the following: 

(a)    The number and kind of shares available for issuance under Article 3, including the numerical share limits in
Articles 3.1 and 3.5; 
 (b)    The number and kind of shares covered by each outstanding Option, SAR, and Restricted
Stock Unit; and/or 
 (c)    The Exercise Price applicable to each outstanding Option and SAR, and the repurchase price,
if any, applicable to Restricted Shares. 
 In the event of a declaration of an extraordinary dividend payable in a form other than Common Shares in an
amount that has a material effect on the price of Common Shares, a recapitalization, 

 
a spin-off or a similar occurrence, the Administrator shall make such adjustments as it, in its sole discretion, deems appropriate to the foregoing. Any
adjustment in the number of shares subject to an Award under this Article 9.1 shall be rounded down to the nearest whole share, although the Administrator in its sole discretion may make a cash payment in lieu of a fractional share. Except as
provided in this Article 9, a Participant shall have no rights by reason of any issuance by the Company of stock of any class or securities convertible into stock of any class, any subdivision or consolidation of shares of stock of any class,
the payment of any stock dividend or any other increase or decrease in the number of shares of stock of any class. 

9.2    Dissolution or Liquidation. To the extent not previously exercised or settled, Options, SARs and
Restricted Stock Units shall terminate immediately prior to the dissolution or liquidation of the Company. 

9.3    Corporate Transactions. In the event that the Company is a party to a merger, consolidation, or a
Change in Control (other than one described in Article 14.6(d)), all Common Shares acquired under the Plan and all Awards outstanding on the effective date of the transaction shall be treated in the manner described in the definitive transaction
agreement (or, in the event the transaction does not entail a definitive agreement to which the Company is party, in the manner determined by the Administrator, with such determination having final and binding effect on all parties), which agreement
or determination need not treat all Awards (or portions thereof) in an identical manner. Unless an Award Agreement provides otherwise, the treatment specified in the transaction agreement or by the Administrator may include (without limitation) one
or more of the following with respect to each outstanding Award: 
 (a)    The continuation of such
outstanding Award by the Company (if the Company is the surviving entity); 
 (b)    The assumption of
such outstanding Award by the surviving entity or its parent, provided that the assumption of an Option or a SAR shall comply with applicable tax requirements; 

(c)    The substitution by the surviving entity or its parent of an equivalent award for such outstanding
Award (including, but not limited to, an award to acquire the same consideration paid to the holders of Common Shares in the transaction), provided that the substitution of an Option or a SAR shall comply with applicable tax requirements; 

(d)    In the case of an Option or SAR, the cancellation of such Award without payment of any
consideration. An Optionee shall be able to exercise his or her outstanding Option or SAR, to the extent such Option or SAR is then vested or become vested as of the effective time of the transaction, during a period of not less than five full
business days preceding the closing date of the transaction, unless (i) a shorter period is required to permit a timely closing of the transaction and (ii) such shorter period still offers the Optionees a reasonable opportunity to exercise
such Option or SAR. Any exercise of such Option or SAR during such period may be contingent on the closing of the transaction; 

(e)    The cancellation of such Award and a payment to the Participant with respect to each share subject
to the portion of the Award that is vested or 

 
becomes vested as of the effective time of the transaction equal to the excess of (A) the value, as determined by the Administrator in its absolute discretion, of the property
(including cash) received by the holder of a Common Share as a result of the transaction, over (if applicable) (B) the per-share Exercise Price of such Award (such excess, if any, the “Spread”). Such payment shall be made
in the form of cash, cash equivalents, or securities of the surviving entity or its parent having a value equal to the Spread. In addition, any escrow, holdback, earn-out or similar provisions in the
transaction agreement may apply to such payment to the same extent and in the same manner as such provisions apply to the holders of Common Shares. If the Spread applicable to an Award (whether or not vested) is zero or a negative number,
then the Award may be cancelled without making a payment to the Participant. In the event that an Award is subject to Code Section 409A, the payment described in this clause (e) shall be made on the settlement date specified in the
applicable Award Agreement, provided that settlement may be accelerated in accordance with Treasury Regulation Section 1.409A-3(j)(4); or 

(f)    The assignment of any reacquisition or repurchase rights held by the Company in respect of an Award
of Restricted Shares to the surviving entity or its parent, with corresponding proportionate adjustments made to the price per share to be paid upon exercise of any such reacquisition or repurchase rights. 

Unless an Award Agreement provides otherwise, each outstanding Award held by a Participant who remains a Service Provider as of the effective time of a
merger, consolidation or Change in Control (other than one described in Article 14.6(d)) (a “Current Participant”) shall become fully vested and, if applicable, exercisable immediately prior to the effective time of the transaction
and, in the case of an Award subject to performance-based vesting conditions, such performance-based vesting conditions shall be deemed achieved at 100% of target levels. However, the prior sentence shall not apply, and an outstanding Award
shall not become vested and, if applicable, exercisable, if and to the extent the Award is continued, assumed or substituted as provided for in clauses (a), (b) or (c) above. In addition, the prior two sentences shall not apply to an
Award held by a Participant who is not a Current Participant unless an Award Agreement provides otherwise or unless the Company and the acquirer agree otherwise. 

For avoidance of doubt, the Administrator shall have the discretion, exercisable either at the time an Award is granted or at any time while the Award remains
outstanding, to provide for the acceleration of vesting upon the occurrence of a Change in Control, whether or not the Award is to be assumed or replaced in the transaction, or in connection with a termination of the Participant’s service
following a transaction. 
 Any action taken under this Article 9.3 shall either preserve an Award’s status as exempt from Code Section 409A or
comply with Code Section 409A. 
 ARTICLE 10.    OTHER AWARDS. 

Subject in all events to the limitations under Article 3 above as to the number of Common Shares available for issuance under this Plan, the
Company may grant other forms of Awards not specifically described herein and may grant awards under other plans or programs, where such awards are settled in the form of Common Shares issued under this Plan. Such Common Shares shall be treated for
all purposes under the Plan like Common Shares issued in settlement of Restricted Stock Units and shall, when issued, reduce the number of Common Shares available under Article 3. 

 ARTICLE 11.    LIMITATION ON RIGHTS. 

11.1    Retention Rights. Neither the Plan nor any Award granted under the Plan shall be deemed to give any
individual a right to remain a Service Provider. The Company and its Parents, Subsidiaries and Affiliates reserve the right to terminate the Service of any Service Provider at any time, with or without cause, subject to applicable laws, the
Company’s certificate of incorporation and by-laws and a written employment agreement (if any). 

11.2    Stockholders’ Rights. Except as set forth in Article 7.4 or 8.4 above, a
Participant shall have no dividend rights, voting rights or other rights as a stockholder with respect to any Common Shares covered by his or her Award prior to the time when a stock certificate for such Common Shares is issued or, if applicable,
the time when he or she becomes entitled to receive such Common Shares by filing any required notice of exercise and paying any required Exercise Price. No adjustment shall be made for cash dividends or other rights for which the record date is
prior to such time, except as expressly provided in the Plan. 
 11.3    Regulatory Requirements. Any
other provision of the Plan notwithstanding, the obligation of the Company to issue Common Shares under the Plan shall be subject to all applicable laws, rules and regulations and such approval by any regulatory body as may be required. The Company
reserves the right to restrict, in whole or in part, the delivery of Common Shares pursuant to any Award prior to the satisfaction of all legal requirements relating to the issuance of such Common Shares, to their registration, qualification or
listing or to an exemption from registration, qualification or listing. The inability of the Company to obtain authority from any regulatory body having jurisdiction, which authority is deemed necessary by the Company’s counsel to be necessary
to the lawful issuance and sale of any Common Shares hereunder, will relieve the Company of any liability in respect of the failure to issue or sell such Common Shares as to which such requisite authority will not have been obtained. 

11.4    Transferability of Awards. The Administrator may, in its sole discretion, permit
transfer of an Award in a manner consistent with applicable law. Unless otherwise determined by the Administrator, Awards shall be transferable by a Participant only by (a) beneficiary designation, (b) a will or (c) the laws of
descent and distribution; provided that, in any event, an ISO may only be transferred by will or by the laws of descent and distribution and may be exercised during the lifetime of the Optionee only by the Optionee or by the Optionee’s guardian
or legal representative. 
 11.5    Recoupment Policy. All Awards granted under the Plan, all
amounts paid under the Plan and all Common Shares issued under the Plan shall be subject to recoupment, clawback or recovery by the Company in accordance with applicable law and with Company policy (whenever adopted) regarding same, whether or not
such policy is intended to satisfy the requirements of the Dodd-Frank Wall Street Reform and Consumer Protection Act, the Sarbanes-Oxley Act, or other applicable law, as well as any implementing regulations and/or listing standards thereunder. 

11.6    Other Conditions and Restrictions on Common Shares. Any Common Shares issued under the Plan shall be
subject to such forfeiture conditions, rights of repurchase, rights of 

 
first refusal, other transfer restrictions and such other terms and conditions as the Administrator may determine. Such conditions and restrictions shall be set forth in the applicable Award
Agreement and shall apply in addition to any restrictions that may apply to holders of Common Shares generally. In addition, Common Shares issued under the Plan shall be subject to such conditions and restrictions imposed either by applicable law or
by Company policy, as adopted from time to time, designed to ensure compliance with applicable law or laws with which the Company determines in its sole discretion to comply including in order to maintain any statutory, regulatory or tax advantage.

 ARTICLE 12.    TAXES. 

12.1    General. It is a condition to each Award under the Plan that a Participant or his or her successor
shall make arrangements satisfactory to the Company for the satisfaction of any federal, state, local or foreign withholding tax obligations that arise in connection with any Award granted under the Plan. The Company shall not be required to issue
any Common Shares or make any cash payment under the Plan unless such obligations are satisfied. 

12.2    Share Withholding. To the extent that applicable law subjects a Participant to tax withholding
obligations, the Administrator may permit such Participant to satisfy all or part of such obligations by having the Company withhold all or a portion of any Common Shares that otherwise would be issued to him or her or by surrendering all or a
portion of any Common Shares that he or she previously acquired. Such Common Shares shall be valued on the date when they are withheld or surrendered. Any payment of taxes by assigning Common Shares to the Company may be subject to restrictions
including any restrictions required by SEC, accounting or other rules. 
 12.3    Section 409A Matters.
Except as otherwise expressly set forth in an Award Agreement, it is intended that Awards granted under the Plan either be exempt from, or comply with, the requirements of Code Section 409A. To the extent an Award is subject to Code
Section 409A (a “409A Award”), the terms of the Plan, the Award and any written agreement governing the Award shall be interpreted to comply with the requirements of Code Section 409A so that the Award is not subject to
additional tax or interest under Code Section 409A, unless the Administrator expressly provides otherwise. A 409A Award shall be subject to such additional rules and requirements as specified by the Administrator from time to time in order for
it to comply with the requirements of Code Section 409A. In this regard, if any amount under a 409A Award is payable upon a “separation from service” to an individual who is considered a “specified employee” (as each term is
defined under Code Section 409A), then no such payment shall be made prior to the date that is the earlier of (i) six months and one day after the Participant’s separation from service or (ii) the Participant’s death, but
only to the extent such delay is necessary to prevent such payment from being subject to Code Section 409A(a)(1). 

12.4    Limitation on Liability. Neither the Company nor any person serving as Administrator shall have any
liability to a Participant in the event an Award held by the Participant fails to achieve its intended characterization under applicable tax law. 

ARTICLE 13.    FUTURE OF THE PLAN. 

13.1    Term of the Plan. The Plan, as set forth herein, shall become effective on the date of its adoption
by the Board, subject to approval of the Company’s stockholders under Article 13.3 below. The Plan shall terminate automatically 10 years after the date when the Board adopted the Plan. 

 13.2    Amendment or Termination. The Board may, at any
time and for any reason, amend or terminate the Plan. No Awards shall be granted under the Plan after the termination thereof. The termination of the Plan, or any amendment thereof, shall not affect any Award previously granted under the Plan. 

13.3    Stockholder Approval. To the extent required by applicable law, the Plan will be subject to the
approval of the Company’s stockholders within 12 months of its adoption date. An amendment of the Plan shall be subject to the approval of the Company’s stockholders only to the extent required by applicable laws, regulations or rules.

 ARTICLE 14.    DEFINITIONS. 

14.1    “Administrator” means the Board or any Committee administering the Plan in accordance with Article
2. 
 14.2    “Affiliate” means any entity other than a Subsidiary, if the Company and/or one or more
Subsidiaries own not less than 50% of such entity. 
 14.3    “Award” means any award granted under the
Plan, including as an Option, a SAR, a Restricted Share award, a Restricted Stock Unit award or another form of equity-based compensation award. 

14.4    “Award Agreement” means a Stock Option Agreement, a SAR Agreement, a Restricted Stock Agreement,
a Restricted Stock Unit Agreement or such other agreement evidencing an Award granted under the Plan. 

14.5    “Board” means the Company’s Board of Directors, as constituted from time to time and, where
the context so requires, reference to the “Board” may refer to a Committee to whom the Board has delegated authority to administer any aspect of this Plan. 

14.6    “Change in Control” means: 

(a)    Any “person” (as such term is used in Sections 13(d) and 14(d) of the Exchange Act) becomes the
“beneficial owner” (as defined in Rule 13d-3 of the Exchange Act), directly or indirectly, of securities of the Company representing more than fifty percent (50%) of the total voting power
represented by the Company’s then-outstanding voting securities; 
 (b)    The consummation of the sale or
disposition by the Company of all or substantially all of the Company’s assets; 
 (c)    The consummation of a
merger or consolidation of the Company with or into any other entity, other than a merger or consolidation which would result in the voting securities of the Company outstanding immediately prior thereto continuing to represent (either by remaining
outstanding or by being converted into voting securities of the surviving entity or its parent) more than fifty percent (50%) of the total voting power represented by the voting securities of the Company or such surviving entity or its parent
outstanding immediately after such merger or consolidation; or 

 (d)    Individuals who are members of the Board (the “Incumbent
Board”) cease for any reason to constitute at least a majority of the members of the Board over a period of 12 months; provided, however, that if the appointment or election (or nomination for election) of any new Board member was approved
or recommended by a majority vote of the members of the Incumbent Board then still in office, such new member shall, for purposes of this Plan, be considered as a member of the Incumbent Board. 

A transaction shall not constitute a Change in Control if its sole purpose is to change the state of the Company’s incorporation or to create a holding
company that will be owned in substantially the same proportions by the persons who held the Company’s securities immediately before such transaction. In addition, if a Change in Control constitutes a payment event with respect to any
Award which provides for a deferral of compensation and is subject to Code Section 409A, then notwithstanding anything to the contrary in the Plan or applicable Award Agreement the transaction with respect to such Award must also constitute a
“change in control event” as defined in Treasury Regulation Section 1.409A-3(i)(5) to the extent required by Code Section 409A. 

14.7    “Code” means the Internal Revenue Code of 1986, as amended. 

14.8    “Committee” means a committee of one or more members of the Board, or of other individuals
satisfying applicable laws, appointed by the Board to administer the Plan. 
 14.9    “Common Share”
means one share of the Company’s common stock. 
 14.10    “Company” means TScan Therapeutics,
Inc., a Delaware corporation. 
 14.11    “Consultant” means a consultant or adviser who provides
bona fide services to the Company, a Parent, a Subsidiary or an Affiliate as an independent contractor and who qualifies as a consultant or advisor under Instruction A.1.(a)(1) of Form S-8 under the
Securities Act. 
 14.12    “Employee” means a common-law
employee of the Company, a Parent, a Subsidiary or an Affiliate. 
 14.13    “Exchange Act” means the
Securities Exchange Act of 1934, as amended. 
 14.14    “Exercise Price,” in the case of an Option,
means the amount for which one Common Share may be purchased upon exercise of such Option, as specified in the applicable Stock Option Agreement. “Exercise Price,” in the case of a SAR, means an amount, as specified in the applicable SAR
Agreement, which is subtracted from the Fair Market Value of one Common Share in determining the amount payable upon exercise of such SAR. 

14.15    “Fair Market Value” means the closing price of a Common Share on any established stock exchange
or a national market system on the applicable date or, if the applicable date is not a trading day, on the last trading day prior to the applicable date, as reported in a source that the Administrator deems reliable. If Common Shares are not traded
on an established stock exchange or a national market system, the Fair Market Value shall be determined by the Administrator in good faith on such basis as it deems appropriate. The Administrator’s determination shall be conclusive and binding
on all persons. Notwithstanding the foregoing, the determination of the Fair Market Value in all cases shall be in accordance with the requirements set forth under Section 409A of the Code to the extent necessary for an Award to comply with, or
be exempt from, Section 409A of the Code. 

 14.16    “IPO Date” means the effective date of the
registration statement filed by the Company with the Securities and Exchange Commission for its initial offering of the Common Shares to the public. 

14.17    “ISO” means an incentive stock option described in Code Section 422(b). 

14.18    “NSO” means a stock option not described in Code Sections 422 or 423. 

14.19    “Option” means an ISO or NSO granted under the Plan and entitling the holder to purchase Common
Shares. 
 14.20    “Optionee” means an individual or estate holding an Option or SAR. 

14.21    “Outside Director” means a member of the Board who is not an Employee. 

14.22    “Parent” means any corporation (other than the Company) in an unbroken chain of corporations
ending with the Company, if each of the corporations other than the Company owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. A corporation that attains the
status of a Parent on a date after the adoption of the Plan shall be considered a Parent commencing as of such date. 

14.23    “Participant” means an individual or estate holding an Award. 

14.24    “Plan” means this TScan Therapeutics, Inc. 2021 Equity Incentive Plan, as amended from time to
time. 
 14.25    “Predecessor Plan” means the Company’s 2018 Stock Plan, as amended. 

14.26    “Restricted Share” means a Common Share awarded under the Plan. 

14.27    “Restricted Stock Agreement” means the agreement consistent with the terms of the Plan between
the Company and the recipient of a Restricted Share that contains the terms, conditions and restrictions pertaining to such Restricted Share. 

14.28    “Restricted Stock Unit” means a bookkeeping entry representing the equivalent of one Common
Share, as awarded under the Plan. 
 14.29    “Restricted Stock Unit Agreement” means the agreement
consistent with the terms of the Plan between the Company and the recipient of a Restricted Stock Unit that contains the terms, conditions and restrictions pertaining to such Restricted Stock Unit. 

14.30    “SAR” means a stock appreciation right granted under the Plan. 

14.31    “SAR Agreement” means the agreement consistent with the terms of the Plan between the Company
and an Optionee that contains the terms, conditions and restrictions pertaining to his or her SAR. 

 14.32    “Securities Act” means the Securities Act of
1933, as amended. 
 14.33    “Service Provider” means any individual who is an Employee, Outside
Director or Consultant, including any prospective Employee, Outside Director or Consultant who has accepted an offer of employment or service and will be an Employee, Outside Director or Consultant after the commencement of their service. 

14.34    “Stock Option Agreement” means the agreement consistent with the terms of the Plan between the
Company and an Optionee that contains the terms, conditions and restrictions pertaining to his or her Option. 

14.35    “Subsidiary” means any corporation (other than the Company) in an unbroken chain of corporations
beginning with the Company, if each of the corporations other than the last corporation in the unbroken chain owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such
chain. A corporation that attains the status of a Subsidiary on a date after the adoption of the Plan shall be considered a Subsidiary commencing as of such date 

14.36    “Substitute Awards” means Awards or Common Shares issued by the Company in assumption of, or
substitution or exchange for, awards previously granted, or the right or obligation to make future awards, in each case by a corporation acquired by the Company or any Affiliate or with which the Company or any Affiliate combines to the extent
permitted by the applicable exchange listing standards.EX-10.4

 Exhibit 10.4 

TSCAN THERAPEUTICS, INC. 

2021 EMPLOYEE STOCK PURCHASE PLAN 

(AS ADOPTED ON APRIL 22, 2021) 

 TSCAN THERAPEUTICS, INC. 

2021 EMPLOYEE STOCK PURCHASE PLAN 

SECTION 1.    PURPOSE OF THE PLAN. 

The Board adopted the Plan effective as of the IPO Date. The purpose of the Plan is to provide Eligible Employees with an opportunity to
increase their proprietary interest in the success of the Company by purchasing Stock from the Company on favorable terms and to pay for such purchases through payroll deductions or other approved contributions. 

SECTION 2.    ADMINISTRATION OF THE PLAN. 

(a)    General. The Plan may be administered by the Board or one or more Committees to which the Board (or an
authorized Board committee) has delegated authority. If administration is delegated to a Committee, the Committee shall have the powers theretofore possessed by the Board, including, to the extent permitted by applicable law, the power to delegate
to a sub-committee any of the administrative powers the Committee is authorized to exercise (and references in this Plan to either the Board or the Administrator shall hereafter also encompass the Committee or
subcommittee, as applicable). The Board may abolish the Committee’s delegation at any time and the Board shall at all times also retain the authority it has delegated to the Committee. Each Committee shall comply with rules and regulations
applicable to it, including under the rules of any exchange on which the Stock is traded, and shall have the authority and be responsible for such functions as have been assigned to it. 

(b)    Powers of the Administrator. Subject to the terms of the Plan, and in the case of a Committee, subject to
the specific duties delegated to the Committee, the Administrator shall have the power to establish the terms and conditions of Offering Periods (which need not be identical) under the Plan, to interpret the Plan and make all other policy decisions
relating to the operation of the Plan. The Administrator may adopt such rules, guidelines and forms as it deems appropriate to implement the Plan. 

(c)    Effects of Administrator’s Decisions. The Administrator’s decisions, determinations and
interpretations shall be final and binding on all interested parties. 
 (d)    Governing Law. The Plan shall be
governed by, and construed in accordance with, the laws of the State of Delaware (except its choice of law provisions). 
 SECTION
3.    STOCK OFFERED UNDER THE PLAN. 
 (a)    Authorized Shares. The number
of shares of Stock available for purchase under the Plan shall be 2,086,000 shares of the Company’s Stock (subject to adjustment pursuant to Subsection (c) below), plus the additional shares described in Subsection (b) below. Shares
of Stock issued pursuant to the Plan may be authorized but unissued shares or treasury shares. 

 (b)    Annual Increase in Shares. On the first day of each fiscal
year of the Company during the term of the Plan, commencing on January 1, 2022 and ending on (and including) January 1, 2041, the aggregate number of shares of Stock that may be issued under the Plan shall automatically increase by a
number equal to the least of (i) one percent (1%) of the total number of shares of Stock actually issued and outstanding on the last day of the preceding fiscal year, or (ii) a number of shares of Stock determined by the Board. 

(c)    Anti-Dilution Adjustments.    In the event that any dividend or other distribution
(whether in the form of cash, stock or other securities or other property), recapitalization, stock split, reverse stock split, reorganization, merger, consolidation, split-up,
spin-off, combination, reclassification, repurchase, or exchange of Stock or other securities of the Company, or other similar change in the corporate structure of the Company affecting the Stock and effected
without receipt or payment of consideration by the Company occurs, then in order to prevent dilution or enlargement of the benefits or potential benefits intended to be made available under the Plan, there will be a proportionate adjustment of the
number and class of Stock that may be delivered under the Plan, the Purchase Price per share and the number of shares and class of Stock covered by each option under the Plan which has not yet been exercised, and the numerical limits of
Sections 3(a), 3(b)(ii) and 9(c). 
 (d)    Reorganizations. In the event of a Corporate Reorganization, the
outstanding rights to purchase Stock under any Offering Period then in progress may be continued, assumed or substituted by the surviving entity or its parent. If such acquirer refuses to continue, assume or substitute for any such rights, then a
new Purchase Date for such Offering Period(s) will be set prior to the effective time of the Corporate Reorganization, the Participants’ accumulated contributions will be applied to purchase Stock on such date, and any such Offering Periods
shall terminate immediately after such purchase. In the event a new Purchase Date is set under this Section 3(d), Participants will be given notice of the new Purchase Date. The Plan shall in no event be construed to restrict in any way the
Company’s right to undertake a dissolution, liquidation, merger, consolidation or other reorganization. 
 SECTION
4.    ENROLLMENT AND PARTICIPATION. 
 (a)    Offering Periods and Purchase
Periods. 
  

	 	(i)	 Base Offering Periods. The Administrator may from time to time establish Offering Periods (consisting of
one or more Purchase Periods) of such frequency and duration as it may deem appropriate (the “Base Offering Periods”); provided that a Base Offering Period shall in no event be longer than 27 months (or such other period as
may be imposed under applicable tax law). Each Base Offering Period shall contain such terms and conditions (consistent with the Plan) as the Administrator deems appropriate. Within the limits of the Plan, the Administrator may change the frequency,
duration and other terms and conditions of the Base Offering Periods as it deems appropriate from time to time. The Base Offering Periods are intended to qualify under Code Section 423. 

	 	(ii)	 Additional Offering Periods. At the discretion of the Administrator, additional Offering Periods (the
“Additional Offering Periods”) may be conducted under the Plan including, if necessary or advisable in the sole discretion of the Administrator, under a separate sub-plan or sub-plans, permitting grants to Eligible Employees of certain Participating Companies (each, a “Sub-Plan”). Such Additional Offering Periods will be
designed to achieve desired tax objectives in particular locations outside the United States or to comply with local laws applicable to offerings in such foreign jurisdictions and may, but need not, qualify under Code Section 423. The
Administrator shall determine the commencement and duration of each Additional Offering Period, and Additional Offering Periods may be consecutive or overlapping. The other terms and conditions of each Additional Offering Period shall be those set
forth in this Plan document or in terms and conditions approved by the Administrator with respect to such Additional Offering Period (whether or not set forth in a written Sub-Plan), with such changes or
additional features as the Administrator determines necessary to comply with local law. Each Additional Offering Period (whether or not set forth in a written Sub-Plan) shall be considered a separate plan from
the Plan (the “Statutory Plan”). The total number of Shares authorized to be issued under the Plan as provided in Section 3 above applies in the aggregate to both the Statutory Plan and any Additional Offering Period.
Unless otherwise superseded by the terms and conditions approved by the Administrator with respect to an Additional Offering Period, the provisions of this Plan document shall govern the operation of any offering conducted hereunder.

  

	 	(iii)	 Separate Offerings. Each Base Offering Period and each Additional Offering Period conducted under the
Plan is intended to constitute a separate “offering” for purposes of Code Section 423. 

  

	 	(iv)	 Equal Rights and Privileges. To the extent an Offering Period is intended to qualify under Code
Section 423, all participants in such Offering Period shall have the same rights and privileges with respect to their participation in such Offering Period in accordance with Code Section 423 and the regulations thereunder except for
differences that may be mandated by local law and are consistent with the requirements of Code Section 423(b)(5). 

(b)    Enrollment. In the case of any individual who qualifies as an Eligible Employee on the first day of any
Offering Period, he or she may elect to become a Participant on such day by filing the prescribed enrollment form with the Company. The enrollment form shall be filed in the prescribed manner during the applicable Enrollment Period for such Offering
Period. The Administrator may establish other procedures for enrollment by Eligible Employees. 
 (c)    Duration of
Participation. Once enrolled in the Plan, a Participant shall continue to participate in the Plan until he or she: 
  

	 	(i)	 Reaches the end of the Offering Period or Purchase Period, as applicable, in which his or her employee
contributions were discontinued under Section 5(c) or 9(b); 

	 	(ii)	 Withdraws from the Plan under Section 6(a); or 

 

	 	(iii)	 Ceases to be an Eligible Employee. 

A Participant whose employee contributions were discontinued automatically under Section 9(b) shall automatically resume participation as described
therein. In all other cases, a former Participant may again become a Participant, if he or she then is an Eligible Employee, by following the procedure described in Subsection (b) above. 

(d)    Applicable Offering Period. For purposes of calculating the Purchase Price under Section 8(b), the
applicable Offering Period shall be determined as follows: 
  

	 	(i)	 Once a Participant is enrolled in the Plan for an Offering Period, such Offering Period shall continue to apply
to him or her until the earliest of (A) the end of such Offering Period, (B) the end of his or her participation under Subsection (c) above, or (C) re-enrollment for a subsequent Offering
Period under Paragraph (ii) or (iii) below. 

  

	 	(ii)	 Any other provision of the Plan notwithstanding, the Administrator (at its sole discretion) may determine prior
to the commencement of any new Offering Period that all Participants shall be re-enrolled for such new Offering Period. In addition, the Administrator may structure an Offering Period so that in the event that
the Fair Market Value of a Share on the first day of the Offering Period for which the Participant is enrolled is higher than on the first day of any subsequent Offering Period, the Participant shall automatically be
re-enrolled for such subsequent Offering Period. 

  

	 	(iii)	 When a Participant reaches the end of an Offering Period but his or her participation is to continue, then such
Participant shall automatically be re-enrolled for the Offering Period that commences immediately after the end of the prior Offering Period. 

SECTION 5.    EMPLOYEE CONTRIBUTIONS. 

(a)    Commencement of Payroll Deductions. A Participant may purchase shares of Stock under the Plan by means of
payroll deductions or (if so approved by the Administrator with respect to all Participants in a Base Offering Period) other approved contributions in form and substance satisfactory to the Administrator. Payroll deductions or other approved
contributions shall commence as soon as reasonably practicable after the Company has received the prescribed enrollment form. In jurisdictions where payroll deductions are not permitted under local law, Participants may purchase shares of Stock by
making contributions in the form that is acceptable and approved by the Administrator. 

 (b)    Amount of Payroll Deductions. An Eligible Employee shall
designate on the prescribed enrollment form the portion of his or her Compensation that he or she elects to have withheld for the purchase of Stock. Such portion shall be a whole percentage of the Eligible Employee’s Compensation, but not less
than 1% nor more than 15% (or such lesser percentage established by the Administrator for an Offering Period). 

(c)    Reducing Withholding Rate or Discontinuing Payroll Deductions. If a Participant wishes to reduce his or her
rate of payroll withholding, such Participant may do so by filing a new enrollment form with the Company in the manner prescribed by the Administrator. The new withholding rate shall be effective as soon as reasonably practicable after the Company
has received such form. The new withholding rate may be 0% or any whole percentage of the Participant’s Compensation, but not more than his or her old withholding rate. The Administrator may limit the number of times a Participant may elect to
reduce his or her rate of withholding during any Offering Period and/or Purchase Period. Unless a different rule is established for an Offering Period, no Participant shall make more than one election under this Subsection (c) during any
Purchase Period. (In addition, employee contributions may be discontinued automatically pursuant to Section 9(b).) 

(d)    Increasing Withholding Rate. Unless the Administrator establishes a different rule for an Offering Period, a
Participant may not increase his or her rate of payroll withholding during a Purchase Period. If a Participant wishes to increase his or her rate of payroll withholding, such Participant may do so by filing a new enrollment form with the Company at
least fifteen (15) calendar days prior to commencement of a Purchase Period (or such other period as is specified by the Administrator). The new withholding rate shall be effective on the first day of the next-upcoming Purchase Period in which
the Participant participates. The new withholding rate may be any whole percentage of the Participant’s Compensation, but not less than 1% nor more than the maximum amount established for the Offering Period. 

SECTION 6.    WITHDRAWAL FROM THE PLAN. 

(a)    Withdrawal. A Participant may elect to withdraw from the Plan (and the Offering Period in which he or she is
participating) by filing the prescribed form with the Company in the prescribed manner at least fifteen (15) calendar days prior to a Purchase Date (or such other period as is specified by the Administrator). As soon as reasonably practicable
thereafter, payroll deductions or other approved contributions shall cease and the entire amount credited to the Participant’s Plan Account with respect to such Offering Period shall be refunded to him or her in cash, without interest (except
as otherwise required by the laws of the local jurisdiction). No partial withdrawals from an Offering Period shall be permitted. 

(b)    Re-Enrollment After Withdrawal. A former Participant who has
withdrawn from the Plan shall not be a Participant until he or she re-enrolls in the Plan under Section 4(b) during an Enrollment Period. Re-enrollment may be
effective only at the commencement of an Offering Period. 

 SECTION 7.    CHANGE IN EMPLOYMENT STATUS. 

(a)    Termination of Employment. Termination of employment as an Eligible Employee for any reason, including death,
shall be treated as an automatic withdrawal from the Plan under Section 6(a). 
 (b)    Transfers of
Employment. If a Participant transfers employment from a Participating Company that is participating in a Base Offering Period to a Participating Company that is participating in an Additional Offering Period, he or she will immediately cease to
participate in the Base Offering Period, as applicable; however, such Participant’s Plan Account will be transferred to the Additional Offering Period, and such Participant will immediately join such Additional Offering Period on the terms and
conditions applicable to such Additional Offering Period, except for any modifications required by applicable law. If a Participant transfers employment from a Participating Company that is participating in an Additional Offering Period to a
Participating Company that is participating in the Base Offering Period, he or she will continue to participate in the Additional Offering Period until the earlier of (i) the end of such Additional Offering Period, or (ii) the commencement
of the first Base Offering Period in which he or she is eligible. If a Participant transfers employment from a Participating Company to a Related Corporation that is not a Participating Company, he or she shall be deemed to have withdrawn from the
Plan pursuant to Section 6(a). 
 (c)    Leave of Absence. For purposes of the Plan, employment shall not be
deemed to terminate when the Participant goes on a military leave, a sick leave or another bona fide leave of absence, if the leave was approved by the Company in writing. Employment, however, shall be deemed to terminate on the first day
following three months after the Participant goes on a leave, unless a contract or statute guarantees his or her right to return to work. Employment shall be deemed to terminate in any event when the approved leave ends, unless the Participant
immediately returns to work. 
 (d)    Death. In the event of the Participant’s death, the amount credited
to his or her Plan Account shall be paid in cash, without interest (unless otherwise required by the laws of the local jurisdiction), to a beneficiary designated by him or her for this purpose on the prescribed form or, if none, to the
Participant’s estate. Such form shall be valid only if it was filed with the Company at the prescribed location before the Participant’s death. 

SECTION 8.    PLAN ACCOUNTS AND PURCHASE OF SHARES. 

(a)    Plan Accounts. The Company shall maintain a Plan Account on its books in the name of each Participant.
Whenever an amount is deducted from the Participant’s Compensation under the Plan, such amount shall be credited to the Participant’s Plan Account. Unless otherwise required by the laws of the local jurisdiction, (i) amounts credited
to Plan Accounts shall not be trust funds and may be commingled with the Company’s general assets and applied to general corporate purposes, and (ii) no interest shall be credited to Plan Accounts. 

(b)    Purchase Price. The Administrator shall establish the Purchase Price for each Offering Period; provided,
however, that the Purchase Price for each share of Stock purchased on a Purchase Date shall not be less than the lower of: 
  

	 	(i)	 85% of the Fair Market Value of such share on the first trading day of such Offering Period; or

	 	(ii)	 85% of the Fair Market Value of such share on the Purchase Date. 

(c)    Number of Shares Purchased. On each Purchase Date, each Participant shall be deemed to have elected to
purchase the number of shares of Stock calculated in accordance with this Subsection (c), unless the Participant has previously elected to withdraw from the Offering Period in accordance with Section 6(a). The amount then in the
Participant’s Plan Account shall be divided by the Purchase Price, and the number of shares that results shall be purchased from the Company with the funds in the Participant’s Plan Account. The foregoing number of shares of Stock that may
be purchased by a Participant are subject to the limitations set forth in Subsection (d) below and in Section 9. The Administrator may determine with respect to all Participants in an Offering Period that any fractional share, as
calculated under this Subsection (c), shall be (i) rounded down to the next lower whole share or (ii) credited as a fractional share. 

(d)    Available Shares Insufficient. In the event that the aggregate number of shares that all Participants elect
to purchase with respect to a particular Purchase Period exceeds (i) the number of shares of Stock that were available under Section 3 above for sale under the Plan on the first day of the applicable Offering Period, or (ii) the
number of shares that were available under Section 3 above for sale under the Plan on the applicable Purchase Date, then the number of shares to which each Participant is entitled shall be determined by multiplying the number of shares
available for issuance by a fraction. The numerator of such fraction is the number of shares that such Participant has elected to purchase, and the denominator of such fraction is the number of shares that all Participants have elected to purchase.
The Company may make a pro rata allocation of the shares available on the first day of an applicable Offering Period pursuant to the preceding sentence, notwithstanding any authorization of additional shares for issuance under the Plan by the
Company’s stockholders subsequent to such date. In the event of a pro-rata allocation under this Section (d), the Administrator may determine in its discretion to continue all Offering Periods then in
effect or terminate all Offering Periods then in effect pursuant to Section 14. 
 (e)    Issuance of Stock.
The shares of Stock purchased by a Participant under the Plan will be registered in the name of such Participant. The Company may permit or require that shares be deposited directly with a broker designated by the Company or to a designated agent of
the Company, and the Company may utilize electronic or automated methods of share transfer. The Company may require that shares be retained with such broker or agent for a designated period of time and/or may establish other procedures to permit
tracking of disqualifying dispositions of such shares. (The two preceding sentences shall apply whether or not the Participant is required to pay income tax in the United States.) 

(f)    Tax Withholding. To the extent required by applicable federal, state, local or foreign law, a Participant
shall make arrangements satisfactory to the Company for the satisfaction of any withholding tax obligations that arise in connection with the Plan. The Company shall not be required to issue any shares of Stock under the Plan until such obligations,
if any, are satisfied. 

 (g)    Unused Cash Balances. Subject to the final sentence of
Section 8(c), any amount remaining in a Participant’s Plan Account at the end of a Purchase Period solely by reason of the inability to purchase a fractional share will be carried over to the next Offering Period or Purchase Period, as
applicable. Any amount remaining in the Participant’s Plan Account that represents the Purchase Price for whole shares that could not be purchased by reason of Subsections (c) or (d) above or Section 9(b) shall be refunded to the
Participant in cash, without interest (except as otherwise required by the laws of the local jurisdiction). 

(h)    Stockholder Approval. Any other provision of the Plan notwithstanding, no shares of Stock shall be purchased
under the Plan unless and until the Company’s stockholders have approved the adoption of the Plan. 
 SECTION
9.    PLAN LIMITATIONS. 
 (a)    Five Percent Limit. Any other provision of
the Plan notwithstanding, no Participant shall be granted a right to purchase Stock under the Plan if, immediately after such right is granted, such Participant would own stock possessing 5% or more of the total combined voting power or value of all
classes of stock of the Company or any Related Corporation, applying the stock attribution rules of Code Section 424(d), and including any stock in which the Participant may purchase under outstanding options as stock owned by such Participant.

 (b)    Dollar Limit. As specified by Code Section 423(b)(8), no Participant shall be entitled to accrue
rights to purchase Stock pursuant to any such rights outstanding under the Plan if and to the extent such accrual, when aggregated with (i) rights to purchase Stock accrued under any other right to purchase Stock under the Plan, and
(ii) similar rights accrued under other employee stock purchase plans (within the meaning of Code Section 423) of the Company or any Related Corporation, would otherwise permit such Participant to purchase more than $25,000 worth of Stock
of the Company or any Related Corporation (determined on the basis of the Fair Market Value per share on the date such rights are granted, and which, with respect to the Plan, will be determined as of the beginning of the respective Offering Period)
for each calendar year such rights are at any time outstanding. 
 If a Participant is precluded by this Subsection (b) from purchasing
additional Stock under the Plan, then his or her employee contributions shall automatically be discontinued and shall automatically resume at the beginning of the next Purchase Period with a scheduled Purchase Date in the next calendar year,
provided that he or she is an Eligible Employee at the beginning of such Purchase Period. 
 (c)    Purchase Period
Share Purchase Limit. The Administrator may establish one or more limits on the number of shares of Stock that may be purchased during any Offering Period and/or Purchase Period, including individual limits and/or aggregate limits. Unless the
Administrator provides otherwise with respect to an Offering Period, any other provision of the Plan notwithstanding, no Participant shall purchase more than 7,500 shares of Stock with respect to any Purchase Period. 

 SECTION 10.    RIGHTS NOT TRANSFERABLE. 

The rights of any Participant under the Plan, or any Participant’s interest in any Stock or moneys to which he or she may be entitled
under the Plan, shall not be transferable by voluntary or involuntary assignment or by operation of law, or in any other manner other than by beneficiary designation or the laws of descent and distribution. If a Participant in any manner attempts to
transfer, assign or otherwise encumber his or her rights or interest under the Plan, other than by beneficiary designation or the laws of descent and distribution, then such act shall be treated as an election by the Participant to withdraw from the
Plan under Section 6(a). 
 SECTION 11.    NO RIGHTS AS AN EMPLOYEE. 

Nothing in the Plan or in any right granted under the Plan shall confer upon the Participant any right to continue in the employ of a
Participating Company for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Participating Companies or of the Participant, which rights are hereby expressly reserved by each, to terminate his or her
employment at any time and for any reason, with or without cause. 
 SECTION 12.    NO RIGHTS AS A STOCKHOLDER.

 A Participant shall have no rights as a stockholder with respect to any shares of Stock that he or she may have a right to purchase under
the Plan until such shares have been purchased on the applicable Purchase Date. 
 SECTION 13.    SECURITIES LAW
REQUIREMENTS. 
 Shares of Stock shall not be issued, and the Company shall have no liability for failure to issue shares of Stock, under
the Plan unless the issuance and delivery of such shares comply with (or are exempt from) all applicable requirements of law, including (without limitation) the Securities Act of 1933, as amended, the rules and regulations promulgated thereunder,
state securities laws and regulations, and the regulations of any stock exchange or other securities market on which the Company’s securities may then be traded. 

SECTION 14.    AMENDMENT OR DISCONTINUANCE. 

(a)    General Rule. The Administrator, in its sole discretion, may amend, suspend, or terminate the Plan, or any
part thereof, at any time and for any reason. If the Plan is terminated, the Administrator, in its discretion, may elect to terminate all outstanding Offering Periods either immediately or upon completion of the purchase of shares of Stock on the
next Purchase Date, or may elect to permit Offering Periods to expire in accordance with their terms (and subject to any adjustment pursuant to Section 3(c) or (d)). If the Offering Periods are terminated prior to expiration, all amounts then
credited to Participants’ accounts which have not been used to purchase shares of Stock will be returned to the Participants (without interest thereon, except as otherwise required by the laws of the local jurisdiction) as soon as
administratively practicable. 

 (b)    Administrator’s Discretion. Without stockholder
consent and without limiting Subsection (a) above, the Administrator will be entitled to change the Offering Periods, limit the frequency and/or number of changes in the amount withheld during an Offering Period, establish the exchange ratio
applicable to amounts withheld in a currency other than U.S. dollars, permit payroll withholding in excess of the amount designated by a Participant in order to adjust for delays or mistakes in the Company’s processing of properly completed
withholding elections, establish reasonable waiting and adjustment periods and/or accounting and crediting procedures to ensure that amounts applied toward the purchase of Stock for each Participant properly correspond with amounts withheld from the
Participant’s Compensation, amend any outstanding purchase rights or clarify any ambiguities regarding the terms of any Offering Period to enable the purchase rights to qualify under and/or comply with Section 423 of the Code, and
establish such other limitations or procedures as it determines in its sole discretion advisable which are consistent with the Plan. The actions of the Administrator pursuant to this paragraph will not be considered to alter or impair the purchase
rights granted under an Offering Period as they are to be deemed part of the initial terms of such Offering Period and purchase rights. 

(c)    Accounting Consideration. In the event the Administrator determines that the ongoing operation of the Plan
may result in unfavorable financial accounting consequences, the Administrator may, in its discretion and, to the extent necessary or desirable, modify, amend or terminate the Plan to reduce or eliminate such accounting consequence including, but
not limited to: 
  

	 	(i)	 Amending the Plan to conform with the safe harbor definition under Financial Accounting Standards Board
Accounting Standards Codification Topic 718 (or successor provision), including with respect to an Offering Period underway at the time; 

  

	 	(ii)	 Altering the Purchase Price for any Offering Period including an Offering Period underway at the time of the
change in Purchase Price; 

  

	 	(iii)	 Shortening any Offering Period (and any Purchase Periods encompassed by such Offering Period) by setting a new
Purchase Date, including with respect to an Offering Period underway at the time of the Administrator’s action; 

  

	 	(iv)	 Reducing the maximum percentage of Compensation a Participant may elect to set aside as payroll deductions; and

  

	 	(v)	 Reducing the maximum number of shares of Stock a Participant may purchase during any Purchase Period.

 Such modifications or amendments will not require stockholder approval or the consent of any Plan Participants. The actions of the
Administrator pursuant to this paragraph will not be considered to alter or impair the purchase rights granted under an Offering Period as they are to be deemed part of the initial terms of such Offering Period and purchase rights. 

(d)    Stockholder Approval. Except as provided in Section 3, any increase in the aggregate number of shares
of Stock that may be issued under the Plan shall be subject to the 

 
approval of the Company’s stockholders. In addition, any other amendment of the Plan shall be subject to the approval of the Company’s stockholders to the extent required under
Section 14(e) or by any applicable law or regulation. 
 (e)    Plan Termination. The Plan shall terminate
automatically 20 years after its adoption by the Board, unless (i) the Plan is extended by the Board and (ii) the extension is approved within 12 months by a vote of the stockholders of the Company. 

SECTION 15.    DEFINITIONS. 

(a)    “Administrator” means the Board or any Committee administering the Plan in accordance with
Section 2. 
 (b)    “Board” means the Board of Directors of the Company, as constituted from time
to time. 
 (c)    “Code” means the Internal Revenue Code of 1986, as amended. 

(d)    “Committee” means a committee of one or more members of the Board, or of other individuals
satisfying applicable laws, appointed by the Board to administer the Plan. 
 (e)    “Company” means
TScan Therapeutics, Inc., a Delaware corporation. 
 (f)    “Compensation” means, unless otherwise
determined by the Administrator with respect to an Offering Period, those components of a Participant’s cash compensation (prior to reductions pursuant to Code Sections 125, 132(f) or 401(k)) that are regular and recurring, including
cash base salary or base hourly pay but excluding any overtime pay or shift differentials, commissions, annual cash incentive compensation, and annual cash bonuses, and further excluding extraordinary cash items (such as one-time bonuses), as well as all non-cash items, moving or relocation allowances,
cost-of-living or tax equalization payments, car allowances, tuition reimbursements, imputed income attributable to cars or life insurance, severance pay, fringe
benefits, contributions or benefits received under employee benefit plans, payments for or related to equity compensation, and any similar items. The Administrator shall determine whether a particular item is included in Compensation. 

(g)    “Corporate Reorganization” means: 

 

	 	(i)	 The consummation of a merger or consolidation of the Company with or into another entity or any other corporate
reorganization; or 

  

	 	(ii)	 The sale, transfer or other disposition of all or substantially all of the Company’s assets or the
complete liquidation or dissolution of the Company. 

 (h)    “Eligible Employee”
means a common law employee of a Participating Company, provided, however, that the Administrator may exclude one or more of the following categories of employees (where exclusion of such employees is permitted by applicable law) from any Offering
Period: (i) employees who have been employed less than two years (or any 

 
shorter period of time established for an Offering Period), (ii) employees who are customarily employed twenty (20) or less hours per week (or any lesser number of hours per week established
for an Offering Period), (iii) employees who are customarily employed for five (5) months or less in a calendar year (or any lesser number of months in a calendar year established for an Offering Period), (iv) “highly compensated
employees” (within the meaning of Code Section 414(q)) or (v) “highly compensated employees” (within the meaning of Code Section 414(q)) with compensation above a certain level and/or who are subject to the disclosure
requirements of Section 16(a) of the Exchange Act. In addition, an individual shall not be considered an Eligible Employee if his or her participation in the Plan is prohibited by the law of any country that has jurisdiction over him or her or
if complying with the laws of the applicable foreign jurisdiction would cause the Plan or an Offering Period to violate the requirements of Code Section 423. With respect to a Base Offering Period, any criteria used to determine Eligible
Employees shall be determined in a manner consistent with Code Section 423. In the case of an Offering Period that is not intended to qualify under Code Section 423, the Administrator may exclude any individual(s) from participation if the
Administrator determines the participation of such individual(s) is not advisable or practicable. 

(i)    “Enrollment Period” means a period prior to the start of an Offering Period during which Eligible
Employees must submit the required enrollment forms to participate in such Offering Period, which period shall end at least five (5) business days (or such other date as may be specified in advance by the Administrator) prior to the start of
the Offering Period. 
 (j)    “Exchange Act” means the Securities Exchange Act of 1934, as amended.

 (k)    “Fair Market Value” means the price at which Stock was last sold in the principal U.S. market
for the Stock on the applicable date or, if the applicable date was not a trading day, on the last trading day prior to the applicable date. If Stock is no longer traded on a public U.S. securities market, the Fair Market Value shall be determined
by the Administrator in good faith on such basis as it deems appropriate. The Administrator’s determination shall be conclusive and binding on all persons. 

(l)    “IPO Date” means the effective date of the registration statement filed by the Company with the
Securities and Exchange Commission for its initial offering of Stock to the public. 
 (m)    “Offering
Period” means any period, including as the context requires the Base Offering Periods and Additional Offering Periods, with respect to which the right to purchase Stock may be granted under the Plan, as determined pursuant to
Section 4(a). 
 (n)    “Participant” means an Eligible Employee who participates in the Plan or
any Sub-Plan, as provided in Section 4. 
 (o)    “Participating
Company” means (i) the Company and (ii) each present or future Subsidiary designated by the Administrator as a Participating Company. 

(p)    “Plan” means this TScan Therapeutics, Inc. 2021 Employee Stock Purchase Plan, as it may be amended
from time to time. 

 (q)    “Plan Account” means the account established for
each Participant pursuant to Section 8(a). 
 (r)    “Purchase Date” means the last trading day of
a Purchase Period. 
 (s)    “Purchase Period” means a period within an Offering Period (which for an
Offering Period with only a single Purchase Period would be coterminous with the Offering Period) during which contributions may be made toward the purchase of Stock under the Plan, as determined pursuant to Section 4(a). 

(t)    “Purchase Price” means the price at which Participants may purchase Stock under the Plan, as
determined pursuant to Section 8(b). 
 (u)    “Related Corporation” means any “parent
corporation” of the Company as defined in Code Section 424(e) or any Subsidiary. 

(v)    “Stock” means the common stock of the Company. 

(w)    “Subsidiary” means any corporation (other than the Company) in an unbroken chain of corporations
beginning with the Company, if each of the corporations other than the last corporation in the unbroken chain owns stock possessing 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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