Exhibit 4.2

0I5N IN&25325$TED 8NDE5 T+E /$WS 2) T+E ST$TE 2) DE/$W$5E CUSIP 60470J 10 3 SEE REVERSE FOR CERTAIN DEFINITIONS This certifies that is the record holder of FULLY PAID AND NONASSESSABLE SHARES OF COMMON STOCK, $0.001 PAR VALUE PER SHARE, OF MIRNA THERAPEUTICS, INC. WUDQVIHUDEOH RQ WKH ERRNV RI WKH FRUSRUDWLRQ LQ SHUVRQ RU E\ GXO\ DXWKRUL]HG DWWRUQH\ XSRQ VXUUHQGHU RI WKLV &HUWL¿FDWH SURSHUO\ HQGRUVHG. 7KLV &HUWL¿FDWH LV QRW YDOLG XQWLO FRXQWHUVLJQHG E\ WKH 7UDQVIHU $JHQW DQG UHJLVWHUHG E\ WKH 5HJLVWUDU. WITNESS WKH IDFVLPLOH VHDO RI WKH &RUSRUDWLRQ DQG WKH IDFVLPLOH VLJQDWXUHV RI LWV GXO\ DXWKRUL]HG RI¿FHUV. Dated: 3UeVLdeQt & &KLe &KLeI )LQaQFLaO 2I¿FeU & SeFUetaU\ SEAL DECEMBER 20, 2007 I E[eFXtLYe 2I¿F eU &28NTE5SI*NED $ND 5E*ISTE5ED: AMERICAN STOCK TRANSFER & TRUST COMPANY, LLC (%522./<N, N<) T5$NS)E5 $*ENT $ND 5E*IST5$5 %<: $8T+25I=ED SI*N$T85E

The Corporation shall furnish without charge to each stockholder who so requests a statement of the powers, designations, preferences and relative, participating, optional or other special rights of each class of stock of the Corporation or series thereof and the qualifications, limitations or restrictions of such preferences and/or rights. Such requests shall be made to the Corporation’s Secretary at the principal office of the Corporation. KEEP THIS CERTIFICATE IN A SAFE PLACE. IF IT IS LOST, STOLEN,OR DESTROYED THE CORPORATION WILL REQUIRE A BOND INDEMNITY AS A CONDITION TO THE ISSUANCE OF A REPLACEMENT CERTIFICATE. 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 TEN ENT JT TEN – – – as tenants in common as tenants by the entireties as joint tenants with right of survivorship and not as tenants in common as community property UNIF GIFT MIN ACT – ......................... Custodian ......................... (Cust) under Uniform Gifts to Minors (Minor) Act.............................................................................. (State) UNIF TRF MIN ACT – ................. Custodian (until age ..................) (Cust) ..................................... under Uniform Transfers (Minor) to Minors Act............................................................ (State) COM PROP – Additional abbreviations may also be used though not in the above list. FOR VALUE RECEIVED, hereby sell(s), assign(s) and transfer(s) unto PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING zIP CODE, OF ASSIGNEE) shares of the capital stock represented by within Certificate, and do hereby irrevocably constitute and appoint attorney-in-fact to transfer the said stock on the books of the within named Corporation with full power of the substitution in the premises. Dated X X Signature(s) Guaranteed: 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 WHATSOEVER. By 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. GUARANTEES BY A NOTARY PUBLIC ARE NOT ACCEPTABLE. SIGNATURE GUARANTEES MUST NOT BE DATED.