EX-10.4





 Exhibit 10.4 

AURA BIOSCIENCES, INC. 

2021 EMPLOYEE STOCK PURCHASE PLAN 

The purpose of the Aura Biosciences, Inc. 2021 Employee Stock Purchase Plan (the “Plan”) is to provide eligible employees of Aura
Biosciences, Inc. (the “Company”) and each Designated Subsidiary (as defined in Section 11) with opportunities to purchase shares of the Company’s common stock, par value $0.00001 per share (the “Common Stock”). An
aggregate of 335,217 shares of Common Stock have been approved and reserved for this purpose, plus on January 1, 2022, and each January 1 thereafter through January 1, 2031, the number of shares of Common Stock reserved and available
for issuance under the Plan shall be cumulatively increased by the least of (i) one percent (1%) of the number of shares of Common Stock issued and outstanding on the immediately preceding December 31st, (ii) 335,217 shares of Common Stock or
(iii) such number of shares of Common Stock as determined by the Administrator (as defined in Section 1). The Plan includes two components: a Code Section 423 Component (the “423 Component”) and a non-Code Section 423 Component (the “Non-423 Component”). It is intended for the 423 Component to constitute an “employee stock purchase plan” within
the meaning of Section 423(b) of the U.S. Internal Revenue Code of 1986, as amended (the “Code”), and the 423 Component shall be interpreted in accordance with that intent. Under the Non-423
Component, which does not qualify as an “employee stock purchase plan” within the meaning of Section 423(b) of the Code, options will be granted pursuant to rules, procedures or sub-plans
adopted by the Administrator designed to achieve tax, securities laws or other objectives for eligible employees. Except as otherwise provided herein, the Non-423 Component will operate and be administered in
the same manner as the 423 Component. 












 Unless otherwise defined herein, capitalized terms in this Plan shall have the meaning
ascribed to them in Section 11. 
 1. Administration. The Plan will be administered by the person or persons (the
“Administrator”) appointed by the Company’s Board of Directors (the “Board”) for such purpose. The Administrator has authority at any time to: (i) adopt, alter and repeal such rules, guidelines and practices for the
administration of the Plan and for its own acts and proceedings as it shall deem advisable; (ii) interpret the terms and provisions of the Plan; (iii) make all determinations it deems advisable for the administration of the Plan;
(iv) decide all disputes arising in connection with the Plan; and (v) otherwise supervise the administration of the Plan. All interpretations and decisions of the Administrator shall be binding on all persons, including the Company and the
Participants. No member of the Board or individual exercising administrative authority with respect to the Plan shall be liable for any action or determination made in good faith with respect to the Plan or any option granted hereunder. 

2. Offerings. The Company may make one or more offerings to eligible employees to purchase Common Stock under the Plan
(“Offerings”). The Administrator shall determine, in its discretion, when the initial Offering and any subsequent Offering shall occur and the duration of each such Offering, provided that no Offering shall exceed 27 months in duration.

 3. Eligibility. All individuals classified as employees on the payroll records of the Company and each Designated Subsidiary are
eligible to participate in any one or more of the Offerings under the Plan, provided that as of the first day of the applicable Offering (the “Offering Date”) they are customarily employed by the Company or a Designated Subsidiary for more
than 20 hours a week (or such lesser number of hours per week as the Administrator shall determine in advance of an Offering) and have completed such period of service prior to the Offering Date as the Administrator may require (but in no event will
the required period of 

  
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continuous employment be equal to or greater than two (2) years). The Administrator may exclude from participation in the Plan or any Offering employees who are “highly compensated
employees” of the Company or a Designated Subsidiary (within the meaning of Section 414(q) of the Code) or a sub-set of such highly compensated employees. Notwithstanding any other provision herein,
individuals who are not contemporaneously classified as employees of the Company or a Designated Subsidiary for purposes of the Company’s or applicable Designated Subsidiary’s payroll system are not considered to be eligible employees of
the Company or any Designated Subsidiary and shall not be eligible to participate in the Plan. In the event any such individuals are reclassified as employees of the Company or a Designated Subsidiary for any purpose, including, without limitation,
common law or statutory employees, by any action of any third party, including, without limitation, any government agency, or as a result of any private lawsuit, action or administrative proceeding, such individuals shall, notwithstanding such
reclassification, remain ineligible for participation. Notwithstanding the foregoing, the exclusive means for individuals who are not contemporaneously classified as employees of the Company or a Designated Subsidiary on the Company’s or
Designated Subsidiary’s payroll system to become eligible to participate in this Plan is through an amendment to this Plan, duly executed by the Company, which specifically renders such individuals eligible to participate herein. 

4. Participation. 
 (a)
Participants. An eligible employee who is not a Participant in any prior Offering may participate in a subsequent Offering by submitting an enrollment form, which may be electronic, to his or her appropriate payroll location at least 15
business days before the Offering Date (or by such other deadline as shall be established by the Administrator for such Offering). 

  
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 (b) Enrollment. The enrollment form will (a) state a whole percentage or amount
to be deducted from an eligible employee’s Compensation (as defined in Section 11) per pay period, (b) authorize the purchase of Common Stock in each Offering in accordance with the terms of the Plan and (c) specify the exact
name or names in which shares of Common Stock purchased for such individual are to be issued pursuant to Section 10. An employee who does not enroll in accordance with these procedures will be deemed to have waived the right to participate.
Unless a Participant files a new enrollment form or withdraws from the Plan, such Participant’s deductions and purchases will continue at the same percentage or amount of Compensation for future Offerings, provided he or she remains eligible.

 (c) Notwithstanding the foregoing, participation in the Plan will neither be permitted nor be denied contrary to the requirements of the
Code and any applicable law. 
 5. Employee Contributions. Each eligible employee may authorize payroll deductions at a minimum of one
(1) percent up to a maximum of fifteen (15) percent of such employee’s Compensation for each pay period (or such other percentage as the Administrator may establish from time to time before an Offering begins). The Company will
maintain book accounts showing the amount of payroll deductions made by each Participant for each Offering. No interest will accrue or be paid on payroll deductions. 

6. Deduction Changes. Except as may be determined by the Administrator in advance of an Offering, a Participant may not increase or
decrease his or her payroll deduction during any Offering, but may increase or decrease his or her payroll deduction with respect to the next Offering (subject to the limitations of Section 5) by filing a new enrollment form at least fifteen
(15) business days before the next Offering Date (or by such other deadline as shall be established by the Administrator for the Offering). The Administrator may, in advance of any Offering, establish rules permitting a Participant to increase,
decrease or terminate his or her payroll deduction during an Offering. 

  
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 7. Withdrawal. A Participant may withdraw from participation in the Plan by
delivering a written notice of withdrawal to his or her appropriate payroll location at least 15 days before the Exercise Date (or by such other deadline as shall be established by the Administrator for the Offering). The Participant’s
withdrawal will be effective as of the next business day. Following a Participant’s withdrawal, the Company will promptly refund such individual’s entire account balance under the Plan to him or her (after payment for any Common Stock
purchased before the effective date of withdrawal). Partial withdrawals are not permitted. Such an employee may not begin participation again during the remainder of the Offering, but may enroll in a subsequent Offering in accordance with
Section 4. 
 8. Grant of Options. On each Offering Date, the Company will grant to each eligible employee who is then a
Participant in the Plan an option (“Option”) to purchase on the last day of such Offering (the “Exercise Date”), at the Option Price hereinafter provided for, the lowest of (a) a number of shares of Common Stock determined
by dividing such Participant’s accumulated payroll deductions on such Exercise Date by the Option Price (as defined herein), (b) the number of shares determined by dividing $25,000 by the Fair Market Value of the Common Stock on the
Offering Date for such Offering; or (c) such other lesser maximum number of shares as shall have been established by the Administrator in advance of the Offering; provided, however, that such Option shall be subject to the limitations set forth
below. Each Participant’s Option shall be exercisable only to the extent of such Participant’s accumulated payroll deductions on the Exercise Date. The purchase price for each share purchased under each Option (the “Option
Price”) will be 85% (or such greater percentage determined by the Administrator in advance of an Offering) of the Fair Market Value of the Common Stock on the Offering Date or the Exercise Date, whichever is less. 

  
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 Notwithstanding the foregoing, no Participant may be granted an option hereunder if such
Participant, immediately after the option was granted, would be treated as owning stock possessing five (5) percent or more of the total combined voting power or value of all classes of stock of the Company or any Parent or Subsidiary (as
defined in Section 11). For purposes of the preceding sentence, the attribution rules of Section 424(d) of the Code shall apply in determining the stock ownership of a Participant, and all stock which the Participant has a contractual
right to purchase shall be treated as stock owned by the Participant. In addition, no Participant may be granted an Option which permits his or her rights to purchase stock under the Plan, and any other employee stock purchase plan of the Company
and its Parents and Subsidiaries, to accrue at a rate which exceeds $25,000 of the fair market value of such stock (determined on the option grant date or dates) for each calendar year in which the Option is outstanding at any time. The purpose of
the limitation in the preceding sentence is to comply with Section 423(b)(8) of the Code and shall be applied taking Options into account in the order in which they were granted. 

9. Exercise of Option and Purchase of Shares. Each employee who continues to be a Participant in the Plan on the Exercise Date shall be
deemed to have exercised his or her Option on such date and shall acquire from the Company such number of whole shares of Common Stock reserved for the purpose of the Plan as his or her accumulated payroll deductions on such date will purchase at
the Option Price, subject to any other limitations contained in the Plan. Any amount remaining in a Participant’s account at the end of an Offering solely by reason of the inability to purchase a fractional share will be carried forward to the
next Offering; any other balance remaining in a Participant’s account at the end of an Offering will be refunded to the Participant promptly. 

  
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 10. Issuance of Certificates. Certificates or book-entries at the Company’s
transfer agent representing shares of Common Stock purchased under the Plan may be issued only in the name of the employee, in the name of the employee and another person of legal age as joint tenants with rights of survivorship, or in the name of a
broker authorized by the employee to be his, her or their, nominee for such purpose. 
 11. Definitions. 

The term “Compensation” means the regular salary or basic hourly rate of compensation. The Administrator, in its discretion, may
establish a different definition of Compensation for an Offering, which for the Section 423 Component shall apply on a uniform and nondiscriminatory basis. Further, the Administrator will have discretion to determine the application of this
definition to eligible employees outside the United States. 
 The term “Designated Subsidiary” means any present or future
Subsidiary (as defined below) that has been designated by the Board to participate in the Plan. The Board may so designate any Subsidiary, or revoke any such designation, at any time and from time to time, either before or after the Plan is approved
by the stockholders , and may further designate such companies or Participants as participating in the 423 Component or the Non-423 Component. The Board may also determine which Subsidiaries or eligible
employees may be excluded from participation in the Plan, to the extent consistent with Section 423 of the Code or as implemented under the Non-423 Component, and determine which Designated Subsidiary or
Subsidiaries will participate in separate Offerings (to the extent that the Company makes separate Offerings). For purposes of the 423 Component, only the Company and its Subsidiaries may be Designated Subsidiaries; provided, however, that at any
given time, a Subsidiary that is a Designated Subsidiary under the 423 Component will not be a Designated Subsidiary under the Non-423 Component. 

  
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 The term “Fair Market Value of the Common Stock” on any given date means the fair
market value of the Common Stock determined in good faith by the Administrator; provided, however, that if the Common Stock is admitted to quotation on the National Association of Securities Dealers Automated Quotation System (“NASDAQ”),
the NASDAQ Global Market, The New York Stock Exchange or another national securities exchange, the determination shall be made by reference to the closing price on such date. If there is no closing price for such date, the determination shall be
made by reference to the last date preceding such date for which there is a closing price. 
 The term “Parent” means a
“parent corporation” with respect to the Company, as defined in Section 424(e) of the Code. 
 The term
“Participant” means an individual who is eligible as determined in Section 3 and who has complied with the provisions of Section 4. 

The term “Subsidiary” means a “subsidiary corporation” with respect to the Company, as defined in Section 424(f) of
the Code. 
 12. Rights on Termination of Employment. If a Participant’s employment terminates for any reason before the
Exercise Date for any Offering, no payroll deduction will be taken from any pay due and owing to the Participant and the balance in the Participant’s account will be paid to such Participant or, in the case of such Participant’s death, to
his or her designated beneficiary as if such Participant had withdrawn from the Plan under Section 7. An employee will be deemed to have terminated employment, for this purpose, if the corporation that employs


  
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him or her, having been a Designated Subsidiary, ceases to be a Subsidiary, or if the employee is transferred to any corporation other than the Company or a Designated Subsidiary; provided,
however, that if a Participant transfers from an Offering under the 423 Component to an Offering under the Non-423 Component, the exercise of the Participant’s Option will be qualified under the 423
Component only to the extent that such exercise complies with Section 423 of the Code. If a Participant transfers from an Offering under the Non-423 Component to an Offering under the 423 Component, the
exercise of the Participant’s Option will remain non-qualified under the Non-423 Component. An employee will not be deemed to have terminated employment for this
purpose if the employee is on an approved leave of absence for military service or sickness or for any other purpose approved by the Company, if the employee’s right to reemployment is guaranteed either by a statute or by contract or under the
policy pursuant to which the leave of absence was granted or if the Administrator otherwise provides in writing. 
 13. Special Rules and Sub-Plans. Notwithstanding anything herein to the contrary, the Administrator may adopt special rules applicable to the employees of a particular Designated Subsidiary, whenever the Administrator determines that
such rules are necessary or appropriate for the implementation of the Plan in a jurisdiction where such Designated Subsidiary has employees; provided that if such special rules or sub-plans are inconsistent
with the requirements of Section 423(b) of the Code, the employees subject to such special rules or sub-plans will participate in the Non-423 Component. Any special
rules or sub-plans established pursuant to this Section 13 shall, to the extent possible, result in the employees subject to such rules having substantially the same rights as other Participants in the
Plan. 
 14. Optionees Not Stockholders. Neither the granting of an Option to a Participant nor the deductions from his or her pay
shall constitute such Participant a holder of the shares of Common Stock covered by an Option under the Plan until such shares have been purchased by and issued to him or her. 

  
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 15. Rights Not Transferable. Rights under the Plan are not transferable by a
Participant other than by will or the laws of descent and distribution, and are exercisable during the Participant’s lifetime only by the Participant. 

16. Application of Funds. All funds received or held by the Company under the Plan may be combined with other corporate funds and may be
used for any corporate purpose. 
 17. Adjustment in Case of Changes Affecting Common Stock. In the event of a subdivision of
outstanding shares of Common Stock, the payment of a dividend in Common Stock or any other change affecting the Common Stock, the number of shares approved for the Plan and the share limitation set forth in Section 8 shall be equitably or
proportionately adjusted to give proper effect to such event. 
 18. Amendment of the Plan. The Board may at any time and from time to
time amend the Plan in any respect, except that without the approval within 12 months of such Board action by the stockholders, no amendment shall be made increasing the number of shares approved for the Plan or making any other change that would
require stockholder approval in order for the 423 Component of the Plan, as amended, to qualify as an “employee stock purchase plan” under Section 423(b) of the Code. 

19. Insufficient Shares. If the total number of shares of Common Stock that would otherwise be purchased on any Exercise Date plus the
number of shares purchased under previous Offerings under the Plan exceeds the maximum number of shares issuable under the Plan, the shares then available shall be apportioned among Participants in proportion to the amount of payroll deductions
accumulated on behalf of each Participant that would otherwise be used to purchase Common Stock on such Exercise Date. 

  
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 20. Termination of the Plan. The Plan may be terminated at any time by the Board.
Upon termination of the Plan, all amounts in the accounts of Participants shall be promptly refunded. 
 21. Governmental Regulations.
The Company’s obligation to sell and deliver Common Stock under the Plan is subject to obtaining all governmental approvals required in connection with the authorization, issuance, or sale of such stock. 

22. Governing Law. This Plan and all Options and actions taken thereunder shall be governed by, and construed in accordance with the
General Corporation Law of the State of Delaware as to matters within the scope thereof, and as to all other matters shall be governed by and construed in accordance with the internal laws of the Commonwealth of Massachusetts, applied without regard
to conflict of law principles. 
 23. Issuance of Shares. Shares may be issued upon exercise of an Option from authorized but unissued
Common Stock, from shares held in the treasury of the Company, or from any other proper source. 
 24. Tax Withholding. Participation
in the Plan is subject to any required tax withholding on income of the Participant in connection with the Plan. Each Participant agrees, by entering the Plan, that the Company and its Subsidiaries shall have the right to deduct any such taxes from
any payment of any kind otherwise due to the Participant, including shares issuable under the Plan. 

  
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 25. Notification Upon Sale of Shares under the 423 Component. Each Participant
agrees, by entering the 423 Component of the Plan, to give the Company prompt notice of any disposition of shares purchased under the Plan where such disposition occurs within two years after the date of grant of the Option pursuant to which such
shares were purchased or within one year after the date such shares were purchased. 
 26. Effective Date. This Plan shall become
effective upon the date immediately preceding the date upon which the registration statement on Form S-1 that is filed by the Company with respect to its initial public offering is declared effective by the
Securities and Exchange Commission following stockholder approval in accordance with applicable state law, the Company’s bylaws and articles of incorporation, each as amended, and applicable stock exchange rules. 

27. Equal Rights and Privileges. Notwithstanding any provision of the Plan to the contrary, all eligible employees who are granted options
under the Plan with respect to the 423 Component shall have the same rights and privileges as determined in accordance with Section 423 of the Code. 

28. No Right to Continued Service. Neither the Plan nor any compensation paid hereunder will confer on any Participant the right to continue as
an employee or in any other capacity. 
 29. Severability. If any provision of the Plan shall for any reason be held to be invalid or
unenforceable, such invalidity or unenforceability shall not affect any other provision hereof, and the Plan shall be construed as if such invalid or unenforceable provision were omitted. 

30. Entire Plan. This Plan constitutes the entire plan with respect to the subject matter hereof and supersedes any prior plans and respect to
the subject matter hereof. 

  
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