JUNO THERAPEUTICS, INC. 2013 EQUITY INCENTIVE PLAN RESTRICTED STOCK PURCHASE AGREEMENT

Exhibit 10.20

JUNO THERAPEUTICS, INC.

2013 EQUITY INCENTIVE PLAN

RESTRICTED STOCK PURCHASE AGREEMENT

Unless otherwise defined herein, the terms defined in the 2013 Equity Incentive Plan (the “Plan”) shall have the same defined meanings in this Restricted Stock Purchase Agreement (the “Agreement”).

Name:

Address:

The undersigned Participant has been granted a right to purchase Common Stock of the Company, subject to the terms and conditions of the Plan and this Agreement, as follows:

Subject to any accelerated vesting provisions in the Plan, [twenty-five percent (25%) of the Shares subject to this Agreement shall be released from the Company’s Repurchase Option on the one (1) year anniversary of the Vesting Commencement Date, and one forty-eighth (1/48^th) of the Shares subject to this Agreement shall be released each month thereafter on the same day of the month as the Vesting Commencement Date (and if there is no corresponding day, on the last day of the month), subject to Participant continuing to be a Service Provider through each such date.]

Any of the Shares which have not yet been released from the Company’s Repurchase Option are referred to herein as “Unreleased Shares.” The Shares which have been released from the Company’s Repurchase Option shall be delivered to Participant at Participant’s request (see Section 11 of Part II of this Agreement).

YOU MUST EXERCISE THIS RESTRICTED STOCK AWARD BEFORE THE EXPIRATION DATE OR IT WILL TERMINATE AND YOU WILL HAVE NO FURTHER RIGHT TO PURCHASE THE SHARES.

1. Sale of Stock. The Administrator of the Company hereby agrees to sell to the Participant named in the Notice of Grant of Restricted Stock in Part I of this Agreement (“Participant”), and Participant hereby agrees to purchase the number of Shares set forth in the Notice of Grant of Restricted Stock, at the Purchase Price per Share set forth in the Notice of Grant of Restricted Stock (the “Purchase Price”), and subject to the terms and conditions of the Plan, which is incorporated herein by reference. Subject to Section 18(c) of the Plan, in the event of a conflict between the terms and conditions of the Plan and this Agreement, the terms and conditions of the Plan shall prevail.

2. Payment of Purchase Price. Participant herewith delivers to the Company the aggregate Purchase Price for the Shares by cash or check, together with any and all withholding taxes due in connection with the purchase of the Shares.

3. Participant’s Representations. In the event the Shares have not been registered under the Securities Act of 1933, as amended, at the time this Restricted Stock Award is exercised, Participant shall, if required by the Company, concurrently with the exercise of all or any portion of this Restricted Stock Award, deliver to the Company his or her Investment Representation Statement in the form attached hereto as Exhibit A.

4. Lock-Up Period. Participant hereby agrees that Participant shall not offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any Common Stock (or other securities) of the Company or enter into any swap, hedging or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of any Common Stock (or other securities) of the Company held by Participant (other than those included in the registration) for a period specified by the representative of the underwriters of Common Stock (or other securities) of the Company not to exceed one hundred and eighty (180) days following the effective date of any registration statement of the Company filed under the Securities Act (or such other period as may be requested by the Company or the underwriters to accommodate regulatory restrictions on (i) the publication or other distribution of research reports and (ii) analyst recommendations and opinions, including, but not limited to, the restrictions contained in NASD Rule 2711(f)(4) or NYSE Rule 472(f)(4), or any successor provisions or amendments thereto).

Participant agrees to execute and deliver such other agreements as may be reasonably requested by the Company or the underwriter which are consistent with the foregoing or which are necessary to give further effect thereto. In addition, if requested by the Company or the representative of the underwriters of Common Stock (or other securities) of the Company, Participant shall provide, within ten (10) days of such request, such information as may be required by the Company or such representative in connection with the completion of any public offering of the Company’s securities pursuant to a registration statement filed under the Securities Act. The

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obligations described in this Section 4 shall not apply to a registration relating solely to employee benefit plans on Form S-1 or Form S-8 or similar forms that may be promulgated in the future, or a registration relating solely to a Commission Rule 145 transaction on Form S-4 or similar forms that may be promulgated in the future. The Company may impose stop-transfer instructions with respect to the shares of Common Stock (or other securities) subject to the foregoing restriction until the end of said one hundred and eighty (180) day (or other) period. Participant agrees that any transferee of the Restricted Stock Award or shares acquired pursuant to the Restricted Stock Award shall be bound by this Section 4.

5. Non-Transferability of Restricted Stock. This Restricted Stock Award may not be transferred in any manner otherwise than by will or by the laws of descent or distribution and may be exercised during the lifetime of Participant only by Participant. The terms of the Plan and this Agreement shall be binding upon the executors, administrators, heirs, successors and assigns of Participant.

6. Tax Consequences. Participant has reviewed with Participant’s own tax advisors the federal, state, local and foreign tax consequences of this investment and the transactions contemplated by this Agreement. Participant is relying solely on such advisors and not on any statements or representations of the Company or any of its agents. Participant understands that Participant (and not the Company) shall be responsible for Participant’s own tax liability that may arise as a result of the transactions contemplated by this Agreement. Participant understands that Section 83 of the Internal Revenue Code of 1986, as amended (the “Code”), taxes as ordinary income the difference between the purchase price for the Shares and the Fair Market Value of the Shares as of the date any restrictions on the Shares lapse. In this context, “restriction” includes the right of the Company to buy back the Shares pursuant to the Repurchase Option. Participant understands that Participant may elect to be taxed at the time the Shares are purchased rather than when and as the Repurchase Option expires by filing an election under Section 83(b) of the Code with the IRS within thirty (30) days from the date of purchase. The form for making this election is attached as Exhibit B-3 hereto.

THE PARTICIPANT ACKNOWLEDGES THAT IT IS THE PARTICIPANT’S SOLE RESPONSIBILITY AND NOT THE COMPANY’S TO FILE TIMELY THE ELECTION UNDER SECTION 83(b), EVEN IF THE PARTICIPANT REQUESTS THE COMPANY OR ITS REPRESENTATIVES TO MAKE THIS FILING ON THE PARTICIPANT’S BEHALF.

7. Tax Withholding. Pursuant to such procedures as the Administrator may specify from time to time, the Company shall withhold the minimum amount required to be withheld for the payment of income, employment and other taxes which the Company determines must be withheld (the “Withholding Taxes”) with respect to Shares released from the Company’s Repurchase Option by, in the Administrator’s discretion: (i) withholding otherwise deliverable Shares upon release from the Company’s Repurchase Option having a Fair Market Value equal the amount of such Withholding Taxes, (ii) withholding the amount of such Withholding Taxes from Participant’s paycheck(s), (iii) requiring Participant to make appropriate arrangements with the Company (or the Parent or Subsidiary employing or retaining Participant) for the satisfaction of all Withholding Taxes, or (iv) a combination of the foregoing. The Company shall not retain fractional Shares to satisfy any portion of the Withholding Taxes. Accordingly, if any withholding is done through the

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withholding of Shares, Participant shall pay to the Company an amount in cash sufficient to satisfy the remaining Withholding Taxes due and payable as a result of the Company not retaining fractional Shares. Should the Company be unable to procure such cash amounts from Participant, Participant agrees and acknowledges that Participant is giving the Company permission to withhold from Participant’s paycheck(s) an amount equal to the remaining Withholding Taxes due and payable as a result of the Company not retaining fractional Shares. Participant acknowledges and agrees that the Company may refuse to honor the exercise and refuse to deliver the Shares if such withholding amounts are not delivered at the time of purchase.

8. No Guarantee of Continued Service. PARTICIPANT ACKNOWLEDGES AND AGREES THAT THE RELEASE OF SHARES FROM THE REPURCHASE OPTION OF THE COMPANY PURSUANT TO THE VESTING SCHEDULE HEREOF IS EARNED ONLY BY CONTINUING AS A SERVICE PROVIDER AT THE WILL OF THE COMPANY (OR THE PARENT OR SUBSIDIARY EMPLOYING OR RETAINING PARTICIPANT) AND NOT THROUGH THE ACT OF BEING HIRED OR PURCHASING SHARES HEREUNDER. PARTICIPANT FURTHER ACKNOWLEDGES AND AGREES THAT THIS AGREEMENT, THE TRANSACTIONS CONTEMPLATED HEREUNDER AND THE VESTING SCHEDULE SET FORTH HEREIN DO NOT CONSTITUTE AN EXPRESS OR IMPLIED PROMISE OF CONTINUED ENGAGEMENT AS A SERVICE PROVIDER FOR THE VESTING PERIOD, FOR ANY PERIOD, OR AT ALL, AND SHALL NOT INTERFERE IN ANY WAY WITH PARTICIPANT’S RIGHT OR THE RIGHT OF THE COMPANY (OR THE PARENT OR SUBSIDIARY EMPLOYING OR RETAINING PARTICIPANT) TO TERMINATE PARTICIPANT’S RELATIONSHIP AS A SERVICE PROVIDER AT ANY TIME, WITH OR WITHOUT CAUSE.

9. Repurchase Option.

(a) In the event Participant’s continuous status as a Service Provider terminates for any or no reason (including death or Disability), the Company shall, upon the date of such termination (as reasonably fixed and determined by the Company), have an irrevocable, exclusive option for a period of ninety (90) days from such date to repurchase up to that number of Shares which constitute the Unreleased Shares (as defined in Part I of this Agreement) at the Purchase Price per share (the “Repurchase Price”) (the “Repurchase Option”).

(b) The Repurchase Option shall be exercised by the Company by delivering written notice to Participant or Participant’s executor (with a copy to the Escrow Holder (as defined in Section 11)) AND, at the Company’s option, (i) by delivering to Participant or Participant’s executor a check in the amount of the aggregate Repurchase Price, or (ii) by the Company canceling an amount of Participant’s indebtedness to the Company equal to the aggregate Repurchase Price, or (iii) by a combination of (i) and (ii) so that the combined payment and cancellation of indebtedness equals such aggregate Repurchase Price. Upon delivery of such notice and the payment of the aggregate Repurchase Price in any of the ways described above, the Company shall become the legal and beneficial owner of the Unreleased Shares being repurchased and all rights and interests therein or relating thereto, and the Company shall have the right to retain and transfer to its own name the number of Unreleased Shares being repurchased by the Company.

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(c) Whenever the Company shall have the right to repurchase the Unreleased Shares hereunder, the Company may designate and assign one or more employees, officers, directors or shareholders of the Company or other persons or organizations to exercise all or a part of the Company’s Repurchase Option to purchase all or a part of the Unreleased Shares. If the Fair Market Value of the Unreleased Shares to be repurchased on the date of such designation or assignment (the “Repurchase FMV”) exceeds the aggregate Repurchase Price of the Unreleased Shares, then each such designee or assignee shall pay the Company cash equal to the difference between the Repurchase FMV and the aggregate Repurchase Price of Unreleased Shares to be purchased.

(d) If the Company or its assignee does not elect to exercise the Repurchase Option conferred above by giving the requisite notice within ninety (90) days following Participant’s termination as a Service Provider, the Repurchase Option shall terminate.

10. Restriction on Transfer. Except for the escrow described in Section 11 or transfer of the Shares to the Company or its assignees contemplated by this Agreement, none of the Shares or any beneficial interest therein shall be transferred, encumbered or otherwise disposed of in any way until the release of such Shares from the Company’s Repurchase Option in accordance with the provisions of this Agreement, other than by will or the laws of descent and distribution. Any distribution or delivery to be made to Participant under this Agreement shall, if Participant is then deceased, be made to Participant’s designated beneficiary, or if no beneficiary survives Participant, to the administrator or executor of Participant’s estate. Any such transferee must furnish the Company with (a) written notice of his or her status as transferee, and (b) evidence satisfactory to the Company to establish the validity of the transfer and compliance with any laws or regulations pertaining to said transfer.

11. Escrow of Shares.

(a) To ensure the availability for delivery of Participant’s Unreleased Shares upon exercise of the Repurchase Option by the Company, Participant will, upon execution of this Agreement, deliver and deposit with an escrow holder designated by the Company (the “Escrow Holder”) the share certificates representing the Unreleased Shares, together with the Assignment Separate from Certificate (the “Stock Assignment”) duly endorsed in blank, attached hereto as Exhibit B-1. The Unreleased Shares and Stock Assignment shall be held by the Escrow Holder, pursuant to the Joint Escrow Instructions of the Company and Participant attached as Exhibit B-2 hereto, until such time as the Company’s Repurchase Option expires.

(b) The Escrow Holder shall not be liable for any act it may do or omit to do with respect to holding the Unreleased Shares in escrow and while acting in good faith and in the exercise of its judgment.

(c) If the Company or any assignee exercises its Repurchase Option hereunder, the Escrow Holder, upon receipt of written notice of such option exercise from the proposed transferee, shall take all steps necessary to accomplish such transfer. Participant hereby appoints the Escrow Holder with full power of substitution, as Participant’s true and lawful attorney-in-fact with irrevocable power and authority in the name and on behalf of Participant to take any action and execute all documents and instruments, including, without limitation, stock powers which may be necessary to transfer the certificate or certificates evidencing such Unreleased Shares to the Company upon such termination.

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(d) When the Repurchase Option has been exercised or expires unexercised or a portion of the Shares has been released from such Repurchase Option, upon Participant’s request the Escrow Holder shall promptly cause a new certificate to be issued for such released Shares and shall deliver such certificate to the Company or Participant, as the case may be.

(e) Subject to the terms hereof, Participant shall have all the rights of a shareholder with respect to such Shares while they are held in escrow, including without limitation, the right to vote the Shares and receive any cash dividends declared thereon.

(f) In the event of any merger, reorganization, consolidation, recapitalization, separation, liquidation, stock dividend, split-up, share combination, or other change in the corporate structure of the Company affecting the Common Stock, the Shares shall be increased, reduced or otherwise changed, and by virtue of any such change Participant shall in his or her capacity as owner of Unreleased Shares that have been awarded to him or her be entitled to new or additional or different shares of stock, cash or securities (other than rights or warrants to purchase securities); such new or additional or different shares, cash or securities shall thereupon be considered to be “Unreleased Shares” and shall be subject to all of the conditions and restrictions which were applicable to the Unreleased Shares pursuant to this Agreement. If Participant receives rights or warrants with respect to any Unreleased Shares, such rights or warrants may be held or exercised by Participant, provided that until such exercise any such rights or warrants and after such exercise any shares or other securities acquired by the exercise of such rights or warrants shall be considered to be Unreleased Shares and shall be subject to all of the conditions and restrictions which were applicable to the Unreleased Shares pursuant to this Agreement. The Administrator in its absolute discretion at any time may accelerate the vesting of all or any portion of such new or additional shares of stock, cash or securities, rights or warrants to purchase securities or shares or other securities acquired by the exercise of such rights or warrants.

12. Company’s Right of First Refusal. Subject to Section 10, before any Shares held by Participant or any transferee (either being sometimes referred to herein as the “Holder”) may be sold or otherwise transferred (including transfer by gift or operation of law), the Company or its assignee(s) shall have a right of first refusal to purchase the Shares on the terms and conditions set forth in this Section 12 (the “Right of First Refusal”).

(a) Notice of Proposed Transfer. The Holder of the Shares shall deliver to the Company a written notice (the “Notice”) stating: (i) the Holder’s bona fide intention to sell or otherwise transfer such Shares; (ii) the name of each proposed purchaser or other transferee (“Proposed Transferee”); (iii) the number of Shares to be transferred to each Proposed Transferee; and (iv) the bona fide cash price or other consideration for which the Holder proposes to transfer the Shares (the “Offered Price”), and the Holder shall offer the Shares at the Offered Price to the Company or its assignee(s).

(b) Exercise of Right of First Refusal. At any time within thirty (30) days after receipt of the Notice, the Company and/or its assignee(s) may, by giving written notice to the Holder, elect to purchase all, but not less than all, of the Shares proposed to be transferred to any one or more of the Proposed Transferees, at the purchase price determined in accordance with subsection (c) below.

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(c) Purchase Price. The purchase price (“Right of First Refusal Price”) for the Shares purchased by the Company or its assignee(s) under this Section 12 shall be the Offered Price. If the Offered Price includes consideration other than cash, the cash equivalent value of the non-cash consideration shall be determined by the Board in good faith.

(d) Payment. Payment of the Right of First Refusal Price shall be made, at the option of the Company or its assignee(s), in cash (by check), by cancellation of all or a portion of any outstanding indebtedness of the Holder to the Company (or, in the case of repurchase by an assignee, to the assignee), or by any combination thereof within thirty (30) days after receipt of the Notice or in the manner and at the times set forth in the Notice.

(e) Holder’s Right to Transfer. If all of the Shares proposed in the Notice to be transferred to a given Proposed Transferee are not purchased by the Company and/or its assignee(s) as provided in this Section 12, then the Holder may sell or otherwise transfer such Shares to that Proposed Transferee at the Offered Price or at a higher price, provided that such sale or other transfer is consummated within one hundred and twenty (120) days after the date of the Notice, that any such sale or other transfer is effected in accordance with any applicable securities laws and that the Proposed Transferee agrees in writing that the provisions of this Section 12 shall continue to apply to the Shares in the hands of such Proposed Transferee. If the Shares described in the Notice are not transferred to the Proposed Transferee within such period, a new Notice shall be given to the Company, and the Company and/or its assignees shall again be offered the Right of First Refusal before any Shares held by the Holder may be sold or otherwise transferred.

(f) Exception for Certain Family Transfers. Anything to the contrary contained in this Section 12 notwithstanding, the transfer of any or all of the Shares during Participant’s lifetime or on Participant’s death by will or intestacy to Participant’s immediate family or a trust for the benefit of Participant’s immediate family shall be exempt from the provisions of this Section 12. “Immediate Family” as used herein shall mean spouse, lineal descendant or antecedent, father, mother, brother or sister. In such case, the transferee or other recipient shall receive and hold the Shares so transferred subject to the provisions of this Agreement, including but not limited to this Section 12 and Section 9, and there shall be no further transfer of such Shares except in accordance with the terms of this Section 12.

(g) Termination of Right of First Refusal. The Right of First Refusal shall terminate as to any Shares upon the earlier of (i) the first sale of Common Stock of the Company to the general public, or (ii) a Change in Control in which the successor corporation has equity securities that are publicly traded.

13. Restrictive Legends and Stop-Transfer Orders.

(a) Legends. Participant understands and agrees that the Company shall cause the legends set forth below or legends substantially equivalent thereto, to be placed upon any certificate(s) evidencing ownership of the Shares together with any other legends that may be required by the Company or by state or federal securities laws:

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THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 (THE “ACT”) AND MAY NOT BE OFFERED, SOLD OR OTHERWISE TRANSFERRED, PLEDGED OR HYPOTHECATED UNLESS AND UNTIL REGISTERED UNDER THE ACT OR, IN THE OPINION OF COUNSEL SATISFACTORY TO THE ISSUER OF THESE SECURITIES, SUCH OFFER, SALE OR TRANSFER, PLEDGE OR HYPOTHECATION IS IN COMPLIANCE THEREWITH.

THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO CERTAIN RESTRICTIONS ON TRANSFER, A RIGHT OF FIRST REFUSAL, AND A REPURCHASE OPTION HELD BY THE ISSUER OR ITS ASSIGNEE(S) AS SET FORTH IN THE RESTRICTED STOCK PURCHASE AGREEMENT BETWEEN THE ISSUER AND THE ORIGINAL HOLDER OF THESE SHARES, A COPY OF WHICH MAY BE OBTAINED AT THE PRINCIPAL OFFICE OF THE ISSUER. SUCH TRANSFER RESTRICTIONS, RIGHT OF FIRST REFUSAL AND REPURCHASE OPTION ARE BINDING ON TRANSFEREES OF THESE SHARES.

THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO RESTRICTIONS ON TRANSFER FOR A PERIOD OF TIME FOLLOWING THE EFFECTIVE DATE OF THE UNDERWRITTEN PUBLIC OFFERING OF THE COMPANY’S SECURITIES SET FORTH IN AN AGREEMENT BETWEEN THE ISSUER AND THE ORIGINAL HOLDER OF THESE SHARES AND MAY NOT BE SOLD OR OTHERWISE DISPOSED OF BY THE HOLDER PRIOR TO THE EXPIRATION OF SUCH PERIOD WITHOUT THE CONSENT OF THE COMPANY OR THE MANAGING UNDERWRITER.

(b) Stop-Transfer Notices. Participant agrees that, in order to ensure compliance with the restrictions referred to herein, the Company may issue appropriate “stop transfer” instructions to its transfer agent, if any, and that, if the Company transfers its own securities, it may make appropriate notations to the same effect in its own records.

(c) Refusal to Transfer. The Company shall not be required (i) to transfer on its books any Shares that have been sold or otherwise transferred in violation of any of the provisions of this Agreement or (ii) to treat as owner of such Shares or to accord the right to vote or pay dividends to any purchaser or other transferee to whom such Shares shall have been so transferred.

14. Notices. Any notice, demand or request required or permitted to be given by either the Company or Participant pursuant to the terms of this Agreement shall be in writing and shall be deemed given when delivered personally or deposited in the U.S. mail, First Class with postage prepaid, and addressed to the parties at the addresses of the parties set forth at the end of this Agreement or such other address as a party may request by notifying the other in writing.

Any notice to the Escrow Holder shall be sent to the Company’s address with a copy to the other party not sending the notice.

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15. No Waiver. Either party’s failure to enforce any provision or provisions of this Agreement shall not in any way be construed as a waiver of any such provision or provisions, nor prevent that party from thereafter enforcing each and every other provision of this Agreement. The rights granted both parties herein are cumulative and shall not constitute a waiver of either party’s right to assert all other legal remedies available to it under the circumstances.

16. Successors and Assigns. The Company may assign any of its rights under this Agreement to single or multiple assignees, and this Agreement shall inure to the benefit of the successors and assigns of the Company. Subject to the restrictions on transfer herein set forth, this Agreement shall be binding upon Participant and his or her heirs, executors, administrators, successors and assigns. The rights and obligations of Participant under this Agreement may only be assigned with the prior written consent of the Company.

17. Interpretation. Any dispute regarding the interpretation of this Agreement shall be submitted by Participant or by the Company forthwith to the Administrator, which shall review such dispute at its next regular meeting. The resolution of such a dispute by the Administrator shall be final and binding on all parties.

18. Additional Documents. Participant agrees upon request to execute any further documents or instruments necessary or desirable to carry out the purposes or intent of this Agreement.

19. Governing Law; Severability. This Agreement is governed by the internal substantive laws, but not the choice of law rules, of [Washington]. In the event that any provision hereof becomes or is declared by a court of competent jurisdiction to be illegal, unenforceable or void, this Agreement shall continue in full force and effect.

20. Entire Agreement. The Plan is incorporated herein by reference. The Plan and this Agreement (including the exhibits referenced herein) constitute the entire agreement of the parties with respect to the subject matter hereof and supersede in their entirety all prior undertakings and agreements of the Company and Participant with respect to the subject matter hereof, and may not be modified adversely to the Participant’s interest except by means of a writing signed by the Company and Participant.

Participant acknowledges receipt of a copy of the Plan and represents that he or she is familiar with the terms and provisions thereof, and hereby accepts this Agreement subject to all of the terms and provisions thereof. Participant has reviewed the Plan and this Agreement in their entirety, has had an opportunity to obtain the advice of counsel prior to executing this Agreement and fully understands all provisions of this Agreement. Participant hereby agrees to accept as binding, conclusive and final all decisions or interpretations of the Administrator upon any questions arising under the Plan or this Agreement. Participant further agrees to notify the Company upon any change in the residence address indicated below.

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EXHIBIT A

INVESTMENT REPRESENTATION STATEMENT

In connection with the purchase of the above-listed Securities, the undersigned Participant represents to the Company the following:

(a) Participant is aware of the Company’s business affairs and financial condition and has acquired sufficient information about the Company to reach an informed and knowledgeable decision to acquire the Securities. Participant is acquiring these Securities for investment for Participant’s own account only and not with a view to, or for resale in connection with, any “distribution” thereof within the meaning of the Securities Act of 1933, as amended (the “Securities Act”).

(b) Participant acknowledges and understands that the Securities constitute “restricted securities” under the Securities Act and have not been registered under the Securities Act in reliance upon a specific exemption therefrom, which exemption depends upon, among other things, the bona fide nature of Participant’s investment intent as expressed herein. In this connection, Participant understands that, in the view of the Securities and Exchange Commission, the statutory basis for such exemption may be unavailable if Participant’s representation was predicated solely upon a present intention to hold these Securities for the minimum capital gains period specified under tax statutes, for a deferred sale, for or until an increase or decrease in the market price of the Securities, or for a period of one year or any other fixed period in the future. Participant further understands that the Securities must be held indefinitely unless they are subsequently registered under the Securities Act or an exemption from such registration is available. Participant further acknowledges and understands that the Company is under no obligation to register the Securities. Participant understands that the certificate evidencing the Securities shall be imprinted with any legend required under applicable state securities laws.

(c) Participant is familiar with the provisions of Rule 701 and Rule 144, each promulgated under the Securities Act, which, in substance, permit limited public resale of “restricted securities” acquired, directly or indirectly from the issuer thereof, in a non-public offering subject to the satisfaction of certain conditions. Rule 701 provides that if the issuer qualifies under Rule 701 at the time of the grant of the Restricted Stock Award to Participant, the exercise shall be exempt from registration under the Securities Act. In the event the Company becomes subject to the reporting requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, ninety (90) days thereafter (or such longer period as any market stand-off agreement may require) the Securities

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exempt under Rule 701 may be resold, subject to the satisfaction of the applicable conditions specified by Rule 144, including in the case of affiliates (1) the availability of certain public information about the Company, (2) the amount of Securities being sold during any three (3) month period not exceeding specified limitations, (3) the resale being made in an unsolicited “broker’s transaction”, transactions directly with a “market maker” or “riskless principal transactions” (as those terms are defined under the Securities Exchange Act of 1934) and (4) the timely filing of a Form 144, if applicable.

In the event that the Company does not qualify under Rule 701 at the time of grant of the Restricted Stock Award, then the Securities may be resold in certain limited circumstances subject to the provisions of Rule 144, which may require (i) the availability of current public information about the Company; (ii) the resale to occur more than a specified period after the purchase and full payment (within the meaning of Rule 144) for the Securities; and (iii) in the case of the sale of Securities by an affiliate, the satisfaction of the conditions set forth in sections (2), (3) and (4) of the paragraph immediately above.

(d) Participant further understands that in the event all of the applicable requirements of Rule 701 or 144 are not satisfied, registration under the Securities Act, compliance with Regulation A, or some other registration exemption shall be required; and that, notwithstanding the fact that Rules 144 and 701 are not exclusive, the Staff of the Securities and Exchange Commission has expressed its opinion that persons proposing to sell private placement securities other than in a registered offering and otherwise than pursuant to Rules 144 or 701 shall have a substantial burden of proof in establishing that an exemption from registration is available for such offers or sales, and that such persons and their respective brokers who participate in such transactions do so at their own risk. Participant understands that no assurances can be given that any such other registration exemption shall be available in such event.

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EXHIBIT B-1

ASSIGNMENT SEPARATE FROM CERTIFICATE

FOR VALUE RECEIVED I,                                 , hereby sell, assign and transfer unto Juno Therapeutics, Inc.                      shares of the Common Stock of Juno Therapeutics, Inc. standing in my name on the books of said corporation represented by Certificate No.          herewith and do hereby irrevocably constitute and appoint                      to transfer the said stock on the books of the within named corporation with full power of substitution in the premises.

This Stock Assignment may be used only in accordance with the Restricted Stock Purchase Agreement between Juno Therapeutics, Inc. and the undersigned dated                     ,          (the “Agreement”).

INSTRUCTIONS: Please do not fill in any blanks other than the signature line. The purpose of this assignment is to enable the Company to exercise its “repurchase option,” as set forth in the Agreement, without requiring additional signatures on the part of the Participant.

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EXHIBIT B-2

JOINT ESCROW INSTRUCTIONS

                    ,         

Corporate Secretary

Juno Therapeutics, Inc.

[Address]

Dear                     :

As Escrow Agent for both Juno Therapeutics, Inc. (the “Company”), and the undersigned purchaser of stock of the Company (the “Participant”), you are hereby authorized and directed to hold the documents delivered to you pursuant to the terms of that certain Restricted Stock Purchase Agreement (the “Agreement”) between the Company and the undersigned, in accordance with the following instructions:

1. In the event the Company and/or any assignee of the Company (referred to collectively for convenience herein as the “Company”) exercises the Company’s repurchase option set forth in the Agreement, the Company shall give to Participant and you a written notice specifying the number of shares of stock to be purchased, the purchase price, and the time for a closing hereunder at the principal office of the Company. Participant and the Company hereby irrevocably authorize and direct you to close the transaction contemplated by such notice in accordance with the terms of said notice.

2. At the closing, you are directed (a) to date the stock assignments necessary for the transfer in question, (b) to fill in the number of shares being transferred, and (c) to deliver the stock assignments, together with the certificate evidencing the shares of stock to be transferred, to the Company or its assignee, against the simultaneous delivery to you of the purchase price (by cash, a check, or some combination thereof) for the number of shares of stock being purchased pursuant to the exercise of the Company’s repurchase option.

3. Participant irrevocably authorizes the Company to deposit with you any certificates evidencing shares of stock to be held by you hereunder and any additions and substitutions to said shares as defined in the Agreement. Participant does hereby irrevocably constitute and appoint you as Participant’s attorney-in-fact and agent for the term of this escrow to execute with respect to such securities all documents necessary or appropriate to make such securities negotiable and to complete any transaction herein contemplated, including but not limited to the filing with any applicable state blue sky authority of any required applications for consent to, or notice of transfer of, the securities. Subject to the provisions of this paragraph 3, Participant shall exercise all rights and privileges of a stockholder of the Company while the stock is held by you.

4. Upon written request of the Participant, but no more than once per calendar year, unless the Company’s repurchase option has been exercised, you shall deliver to Participant a certificate or certificates representing so many shares of stock as are not then subject to the

Company’s repurchase option. Within one hundred and twenty (120) days after cessation of Participant’s continuous employment by or services to the Company, or any parent or subsidiary of the Company, you shall deliver to Participant a certificate or certificates representing the aggregate number of shares held or issued pursuant to the Agreement and not purchased by the Company or its assignees pursuant to exercise of the Company’s repurchase option.

5. If at the time of termination of this escrow you should have in your possession any documents, securities, or other property belonging to Participant, you shall deliver all of the same to Participant and shall be discharged of all further obligations hereunder.

6. Your duties hereunder may be altered, amended, modified or revoked only by a writing signed by all of the parties hereto.

7. You shall be obligated only for the performance of such duties as are specifically set forth herein and may rely and shall be protected in relying or refraining from acting on any instrument reasonably believed by you to be genuine and to have been signed or presented by the proper party or parties. You shall not be personally liable for any act you may do or omit to do hereunder as Escrow Agent or as attorney-in-fact for Participant while acting in good faith, and any act done or omitted by you pursuant to the advice of your own attorneys shall be conclusive evidence of such good faith.

8. You are hereby expressly authorized to disregard any and all warnings given by any of the parties hereto or by any other person or corporation, excepting only orders or process of courts of law and are hereby expressly authorized to comply with and obey orders, judgments or decrees of any court. In case you obey or comply with any such order, judgment or decree, you shall not be liable to any of the parties hereto or to any other person, firm or corporation by reason of such compliance, notwithstanding any such order, judgment or decree being subsequently reversed, modified, annulled, set aside, vacated or found to have been entered without jurisdiction.

9. You shall not be liable in any respect on account of the identity, authorities or rights of the parties executing or delivering or purporting to execute or deliver the Agreement or any documents or papers deposited or called for hereunder.

10. You shall not be liable for the outlawing of any rights under the Statute of Limitations with respect to these Joint Escrow Instructions or any documents deposited with you.

11. You shall be entitled to employ such legal counsel and other experts as you may deem necessary properly to advise you in connection with your obligations hereunder, may rely upon the advice of such counsel, and may pay such counsel reasonable compensation therefor.

12. Your responsibilities as Escrow Agent hereunder shall terminate if you shall cease to be an officer or agent of the Company or if you shall resign by written notice to each party. In the event of any such termination, the Company shall appoint a successor Escrow Agent.

13. If you reasonably require other or further instruments in connection with these Joint Escrow Instructions or obligations in respect hereto, the necessary parties hereto shall join in furnishing such instruments.

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14. It is understood and agreed that should any dispute arise with respect to the delivery and/or ownership or right of possession of the securities held by you hereunder, you are authorized and directed to retain in your possession without liability to anyone all or any part of said securities until such disputes shall have been settled either by mutual written agreement of the parties concerned or by a final order, decree or judgment of a court of competent jurisdiction after the time for appeal has expired and no appeal has been perfected, but you shall be under no duty whatsoever to institute or defend any such proceedings.

15. Any notice required or permitted hereunder shall be given in writing and shall be deemed effectively given upon personal delivery or upon deposit in the United States Post Office, by registered or certified mail with postage and fees prepaid, addressed to each of the other parties thereunto entitled at the following addresses or at such other addresses as a party may designate by ten (10) days advance written notice to each of the other parties hereto.

16. By signing these Joint Escrow Instructions, you become a party hereto only for the purpose of said Joint Escrow Instructions; you do not become a party to the Agreement.

17. This instrument shall be binding upon and inure to the benefit of the parties hereto, and their respective successors and permitted assigns.

18. These Joint Escrow Instructions shall be governed by the internal substantive laws, but not the choice of law rules, of [Washington].

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EXHIBIT B-3

ELECTION UNDER SECTION 83(b)

OF THE INTERNAL REVENUE CODE OF 1986

The undersigned taxpayer hereby elects, pursuant to Sections 55 and 83(b) of the Internal Revenue Code of 1986, as amended, to include in taxpayer’s gross income or alternative minimum taxable income, as the case may be, for the current taxable year the amount of any compensation taxable to taxpayer in connection with taxpayer’s receipt of the property described below.

The Shares may not be transferred and are subject to forfeiture under the terms of an agreement between the taxpayer and the Company. These restrictions lapse upon the satisfaction of certain conditions contained in such agreement.

The undersigned has submitted a copy of this statement to the person for whom the services were performed in connection with the undersigned’s receipt of the above-described property. The transferee of such property is the person performing the services in connection with the transfer of said property.

The undersigned understands that the foregoing election may not be revoked except with the consent of the Commissioner.