Exhibit 10.16(c)

EXECUTION COPY


AGREEMENT REGARDING

SENIOR SECURED
PROMISSORY NOTE


                This
AGREEMENT REGARDING SENIOR SECURED PROMISSORY NOTE (“Agreement”), dated
effective as of August  23,
2006 (the “Effective Date”), is entered into by and between Genzyme
Corporation, with its principal office at 500 Kendall Street, Cambridge, MA
02142 (“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 Seven
Million Dollars ($7,000,000) or such lesser amount as advanced to Dyax pursuant
to the terms and conditions of a Senior Secured Promissory Note, also dated May
31, 2002 (the “Note”);


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


                WHEREAS,
under the terms of the Note, the principal amount, together with all accrued
and unpaid interest due thereunder, was originally due and payable on the May
31, 2005 (the “Maturity Date”);


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


                WHEREAS,
the conditions set forth Schedule A being satisfied on the Maturity Date, Dyax
exercised 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 Maturity
Date, among other things, and to restate the Note on the terms and conditions
set 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 of
which are hereby acknowledged, Genzyme and Dyax hereby agree as follows:


                1.             Termination of Securitv Agreement; Release of
Collateral. 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 result
of such termination, Genzyme agrees to take any and all reasonable action
necessary to promptly release all Collateral (as defined therein) currently
pledged to secure the obligations of Dyax under the Security Agreement.


                2.             Amended and Restated Promissory Note. The
Note is hereby amended and restated in its entirety, as set forth on Exhibit
A attached hereto.


                3.             Letter of Credit. In place of the
Collateral, Dyax is concurrently delivering a letter of credit in the form
attached here to as Exhibit B.


                IN
WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed
by their respective duly authorized representatives, under seal, as of the
Effective 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