THIS CONVERTIBLE PROMISSORY NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. NO SALE OR DISPOSITION MAY BE EFFECTED EXCEPT IN COMPLIANCE WITH RULE 144 UNDER SAID ACT OR AN EFFECTIVE REGISTRATION STATEMENT RELATED THERETO OR AN OPINION OF COUNSEL FOR THE HOLDER SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED UNDER THE ACT OR RECEIPT OF A NO-ACTION LETTER FROM THE SECURITIES AND EXCHANGE COMMISSION. Convertible Promissory' Note $XXX,XXX TBD Cambridge, MA For value received OH2 Laboratories, LLC, a Delaware limited liability company (the "Company"), promises to pay to or his assigns ("Holder") the principal sum of US$XXX,XXX together with accrued and unpaid interest thereon, each due and payable on the date and in the manner set forth below. This convertible promissory note (the "Note") is issued as part of a series of similar convertible promissory notes (collectively, the "Notes") pursuant to the terms of that certain Convertible Promissory Note Purchase Agreement (as amended, the "Agreement") dated as of March 9, 2015 to the persons and entities listed on Schedule A to the Agreement (collectively, the "Holders"). Capitalized terms used in this Note and not otherwise defined will have the meanings given to them in the Agreement. I. Repayment. All payments of interest and principal will be in lawful money of the United States of America and will be made pro rata among all Holders. All payments will be applied first to accrued interest, and thereafter to principal. The outstanding principal amount of the Loan will be due and payable on May 30, 2016 (the "Maturity Date"). 2. Interest Rate. The Company promises to pay simple interest on the outstanding principal amount of this Note from the date of this Note until payment in full, which interest will be payable at the annual rate of 5%. Interest will be due and payable on the Maturity Date and will be calculated on the basis of a 365-day year fo