**Exhibit 10.16(c)EXECUTION COPY**

**AGREEMENT REGARDING****SENIOR SECUREDPROMISSORY NOTE**

                ThisAGREEMENT REGARDING SENIOR SECURED PROMISSORY NOTE (“Agreement”), datedeffective as of August  23,2006 (the “Effective Date”), is entered into by and between GenzymeCorporation, with its principal office at 500 Kendall Street, Cambridge, MA02142 (“Genzyme”), and Dyax Corp., with a principal office at 300 Technology Square,Cambridge, Massachusetts 02139 (“Dyax”).

                WHEREAS,on May 31, 2002, Genzyme agreed to loan Dyax up the principal amount of SevenMillion Dollars ($7,000,000) or such lesser amount as advanced to Dyax pursuantto the terms and conditions of a Senior Secured Promissory Note, also dated May31, 2002 (the “Note”);

                WHEREAS,as of the date hereof, Dyax has borrowed the entire principal amount of SevenMillion Dollars ($7,000,000);

                WHEREAS,under the terms of the Note, the principal amount, together with all accruedand unpaid interest due thereunder, was originally due and payable on the May31, 2005 (the “Maturity Date”);

                WHEREAS,under the terms of the Note, Dyax had the right to extend the Maturity Date toMay 31, 2007 if, on the Maturity Date, Dyax met the conditions described inSchedule A attached to the Note;

                WHEREAS,the conditions set forth Schedule A being satisfied on the Maturity Date, Dyaxexercised its right to extend the Maturity Date to May 31, 2007; and

                WHEREAS,Genzyme and Dyax now wish to amend the Note to further extend the MaturityDate, among other things, and to restate the Note on the terms and conditionsset forth herein.

                NOW,THEREFORE, in consideration of the promises and agreements set forth herein,and for other good and valuable consideration, the receipt and sufficiency ofwhich are hereby acknowledged, Genzyme and Dyax hereby agree as follows:

**                1.            ****Termination of Securitv Agreement; Release ofCollateral****.**The Security Agreement, dated May 31, 2002, by and between Dyax and Genzyme, as amended to date(the “Security Agreement”), is hereby terminated in its entirety. As a resultof such termination, Genzyme agrees to take any and all reasonable actionnecessary to promptly release all Collateral (as defined therein) currentlypledged to secure the obligations of Dyax under the Security Agreement.

**                2.             Amended and Restated Promissory Note.**TheNote is hereby amended and restated in its entirety, as set forth on ExhibitA attached hereto.

**                3.            ****Letter of Credit****.**In place of theCollateral, Dyax is concurrently delivering a letter of credit in the formattached here to as Exhibit B.

                INWITNESS WHEREOF, the parties hereto have caused this Agreement to be executedby their respective duly authorized representatives, under seal, as of theEffective Date hereof.
| **GENZYME CORPORATION** | ** **        | **DYAX CORP.**                               |
|-------------------------|-------------------|----------------------------------------------|
| ** **              | ** **        | ** **                                   |
| ** **              | ** **        | ** **                                   |
| By:                     | /s/ Michael Wyzga |                                         | ** ** | By: | **/**s/ Thomas R. Beck | ** ** |
| Name: Michael Wyzga     | ** **        | Name: Thomas R. Beck                         |
| Title: CFO              | ** **        | Title: President and Chief Operating Officer |
|                         |                   |                                              |  |  |  |  |

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