Exhibit 4.2

 



ZQ|CERT#|COY|CLS|RGSTRY|ACCT#|TRANSTYPE|RUN#|TRANS#   . COMMON STOCK PAR VALUE $0.001 COMMON STOCK Certificate Number ZQ00000000   Shares * * 000000 * * * * * * * * * * * * * * * * * * * * * 000000 * * * * *   * * * * * * * * * * * * * * * * 000000 * * * * * * * * * * * * * * * * * * *   * * 000000 * * * * * * * * * * * * * * * * * * * * * 000000 * * * * * * * * *   * * * * * NEON THERAPEUTICS, INC. INCORPORATED UNDER THE LAWS OF THE STATE OF   DELAWARE ** Mr. Alexander David Sample **** Mr. Alexander David Sample ****   Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander   David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample   **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.   Alexander David SEE REVERSE FOR CERTAIN DEFINITIONS THIS CERTIFIES THAT   Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample ****   Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander   MR. SAMPLE & MRS. SAMPLE & David Sample **** Mr. Alexander David   Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample ****   Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander   David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample   **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.   Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander   David Sample **** Mr. Alexander David Sample **** Mr. Alexander DavidMSampRle   ****.Mr. SAlexaAnderMDavidPSamLple *E*** Mr. &AlexandMer DavRid   SaSmple.**** SMr. AAlexanMder DaPvid SLampEle **** Mr. Alexander David Sample   **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.   Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander   David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample   **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.   Alexander David Sample **** Mr. Alexander David Sample **** Mr. Alexander   David Sample **** Mr. Alexander David Sample **** Mr. Alexander David Sample   **** Mr. Alexander David Sample **** Mr. Alexander David Sample **** Mr.   Alexander David Sample **** Mr. Alexander David Sample **** Mr. Sample ****   Mr. Sample is the owner of **000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares***   *000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****   000000**Shar*es****0*000Z00**SEhareRs****00O0000**ShHares**U**0000N00**SDhares*R***000E000**DShares**T**000H000**SOhares*U***000S000**AShareNs****00D0000**Shares****0   THIS CERTIFICATE IS TRANSFERABLE IN CITIES DESIGNATED BY THE TRANSFER AGENT,   AVAILABLE ONLINE AT www.computershare.com   00000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****00   0000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000   000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****0000   00**Shares****0Z0000E0**ShRares***O*000000*H*ShareUs****0N00000D**SharRes****0E0000D0**ShareAs****0N00000D**SharesZ****00E0000R**SharOes****0*000*00**Shares****00000   0**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000   **Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000*   *Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**   Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**Shares****000000**S   FULLY-PAID AND NON-ASSESSABLE SHARES OF COMMON STOCK OF Neon Therapeutics, Inc.   (hereinafter called the “Company”), transferable on the books of the Company   in person or by duly authorized attorney, upon surrender of this Certificate   properly endorsed. This Certificate and the shares represented hereby, are   issued and shall be held subject to all of the provisions of the Certificate   of Incorporation, as amended, and the By-Laws, as amended, of the Company   (copies of which are on file with the Company and with the Transfer Agent),   to all of which each holder, by acceptance hereof, assents. This Certificate   is not valid unless countersigned and registered by the Transfer Agent and   Registrar. Witness the facsimile seal of the Company and the facsimile   signatures of its duly authorized officers. DATED DD-MMM-YYYY COUNTERSIGNED   AND REGISTERED: COMPUTERSHARE TRUST COMPANY, N.A. TRANSFERAGENT ANDREGISTRAR,   FACSIMILE SIGNATURE TO COME President October 21, 2013 FACSIMILE SIGNATURE TO   COME By Secretary AUTHORIZEDSIGNATURE . CUSIP/IDENTIFIER Holder ID Insurance   Value Number of Shares DTC Certificate Numbers 1234567890/1234567890   1234567890/1234567890 1234567890/1234567890 1234567890/1234567890   1234567890/1234567890 1234567890/1234567890 Total Transaction XXXXXX XX X   XXXXXXXXXX 1,000,000.00 123456 12345678 123456789012345 PO BOX 43004, Providence,   RI 02940-3004 Num/No. Denom. Total 1 2 3 4 5 6 7 1 2 3 4 5 6 1 2 3 4 5 6 MR A   SAMPLE DESIGNATION (IF ANY) ADD 1 ADD 2 ADD 3 ADD 4 CUSIP XXXXXX XX X 


    




 






 



. NEON   THERAPEUTICS, INC. THE COMPANY WILL FURNISH WITHOUT CHARGE TO EACH   SHAREHOLDER WHO SO REQUESTS, A SUMMARY OF THE POWERS, DESIGNATIONS,   PREFERENCES AND RELATIVE, PARTICIPATING, OPTIONAL OR OTHER SPECIAL RIGHTS OF   EACH CLASS OF STOCK OF THE COMPANY AND THE QUALIFICATIONS, LIMITATIONS OR   RESTRICTIONS OF SUCH PREFERENCES AND RIGHTS, AND THE VARIATIONS IN RIGHTS,   PREFERENCES AND LIMITATIONS DETERMINED FOR EACH SERIES, WHICH ARE FIXED BY   THE CERTIFICATE OF INCORPORATION OF THE COMPANY, AS AMENDED, AND THE RESOLUTIONS   OF THE BOARD OF DIRECTORS OF THE COMPANY, AND THE AUTHORITY OF THE BOARD OF   DIRECTORS TO DETERMINE VARIATIONS FOR FUTURE SERIES. SUCH REQUEST MAY BE MADE   TO THE OFFICE OF THE SECRETARY OF THE COMPANY OR TO THE TRANSFER AGENT. THE   BOARD OF DIRECTORS MAY REQUIRE THE OWNER OF A LOST OR DESTROYED STOCK   CERTIFICATE, OR HIS LEGAL REPRESENTATIVES, TO GIVE THE COMPANY A BOND TO   INDEMNIFY IT AND ITS TRANSFER AGENTS AND REGISTRARS AGAINST ANY CLAIM THAT   MAY BE MADE AGAINST THEM ON ACCOUNT OF THE ALLEGED LOSS OR DESTRUCTION OF ANY   SUCH CERTIFICATE. (Cust) (Minor) (State) (Cust) and not as tenants in common   (Minor) (State) PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF   ASSIGNEE For value received, hereby sell, assign and transfer unto (PLEASE PRINT   OR TYPEWRITE NAME AND ADDRESS, INCLUDING POSTAL ZIP CODE, OF ASSIGNEE) Shares   of the common stock represented by the within Certificate, and do hereby   irrevocably constitute and appoint Attorney to transfer the said stock on the   books of the within-named Company with full power of substitution in the   premises. Dated: 20 Signature: Signature: Notice: The signature to this   assignment must correspond with the name as written upon the face of the   certificate, in every particular, without alteration or enlargement, or any   change whatever. The IRS requires that the named transfer agent (“we”) report   the cost basis of certain shares or units acquired after January 1, 2011. If   your shares or units are covered by the legislation, and you requested to   sell or transfer the shares or units using a specific cost basis calculation   method, then we have processed as you requested. If you did not specify a   cost basis calculation method, then we have defaulted to the first in, first   out (FIFO) method. Please consult your tax advisor if you need additional   information about cost basis. If you do not keep in contact with the issuer   or do not have any activity in your account for the time period specified by   state law, your property may become subject to state unclaimed property laws   and transferred to the appropriate state. Signature(s) Guaranteed: Medallion   Guarantee Stamp THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN ELIGIBLE   GUARANTOR INSTITUTION (Banks, Stockbrokers, Savings and Loan Associations and   Credit Unions) WITH MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE MEDALLION   PROGRAM, PURSUANT TO S.E.C. RULE 17Ad-15. The following abbreviations, when   used in the inscription on the face of this certificate, shall be construed   as though they were written out in full according to applicable laws or   regulations: TEN COM - as tenants in common UNIF GIFT MIN ACT -   ............................................Custodian   ................................................ TEN ENT - as tenants by the   entireties under Uniform Gifts to Minors   Act......................................................... JT TEN - as   joint tenants with right of survivorship UNIF TRF MIN ACT -   ............................................Custodian (until age   ................................) .............................under Uniform   Transfers to Minors Act ................... Additional abbreviations may also   be used though not in the above list.