Exhibit 10.1

FOURTH AMENDMENT TO LEASE

THIS FOURTH AMENDMENT TO LEASE (this “ Fourth Amendment ”) is made as of August 25, 2009, by and between ARE-TECH SQUARE, LLC , a Delaware limited liability company (“ Landlord ”), and DYAX CORP. , a Delaware corporation (“ Tenant ”).

RECITALS

A.                                     Landlord and Tenant are now parties to that certain Lease dated June 13, 2001, as amended by that certain First Amendment to Lease dated March 1, 2002, as further amended by that certain letter agreement dated April 17, 2002, as further amended by that certain Second Amendment to Lease dated December 1, 2002 (“ Second Amendment ”), and as further amended by that certain Third Amendment to Lease dated September 6, 2007 (“ Third Amendment ”) (as amended, the “ Lease ”), pursuant to which Tenant leases certain space containing approximately 91,527 rentable square feet (“ Premises ”) in a building located at 300 Technology Square, Cambridge, Massachusetts (the “ Building ”)  The Premises are more particularly described in the Lease.  Capitalized terms used herein without definition shall have the meanings defined for such terms in the Lease.

B.                                     Landlord and Tenant desire to, subject to the terms and conditions set forth herein, among other things, amend the Lease to reflect the surrender of that portion of the Premises containing approximately 24,154 rentable square feet, consisting of (i) approximately 24,122 rentable square feet located on the seventh floor of the Building (“ 7 th  Floor Surrender Premises ”), and (ii) approximately 32 rentable square feet located on the first floor of the Building (“ 1 st  Floor Surrender Premises ”), all as more particularly described on Exhibit A attached to this Fourth Amendment (the 7 th  Floor Surrender Premises and the 1 st  Floor Surrender Premises shall be collectively referred to herein as the “ Surrender Premises ”).  Tenant shall surrender the Surrender Premises to Landlord on September 15, 2009 (“ Surrender Date ”).

NOW, THEREFORE, in consideration of the foregoing Recitals, which are incorporated herein by this reference, the mutual promises and conditions contained herein, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant hereby agree as follows:

1.                                       Premises .

a.                                        Following the Surrender Date .  Commencing on September 16, 2009, the definitions of “ Premises ” contained on page i of Exhibit 1 of the Lease and “ Total rentable square feet of Premises ” contained in Art. 7, page iv of Exhibit 1 of the Lease, are amended and restated in their entirety as follows:

“Total rentable square feet of the Premises:  approximately 67,373 rentable square feet, consisting of (i) approximately 24,122 rentable square feet of space on the 6 th  floor, (ii) approximately 24,122 rentable square feet of space on the 5 th  floor, (iii) approximately 17,476 rentable square feet of space on the 8 th  floor, (iv) approximately 1,445 rentable square feet of space on the 1 st  floor (“ Dyax 1 st  Floor Space ”), and (v) approximately 208 rentable square feet of additional storage space on the 1 st  floor (“ 1 st  Floor Storage Space ”).”

In addition, commencing on September 16, 2009, Exhibit 2 attached to the Lease shall be amended to exclude the Surrender Premises.