exhibit 10. 2 purchase agreement this purchase agreement ( this “ purchase agreement ” ) is made this 31st day of october, 2007 between momenta pharmaceuticals, inc. ( “ seller ” ) and alnylam pharmaceuticals, inc. ( “ buyer ” ). whereas, seller is the subtenant under that certain sublease dated as of september 8, 2006 ( the “ sublease ” ) with archemix corp., as sublandlord ( “ sublandlord ” ) with respect to approximately 22, 364 rentable square feet ( the “ subleased premises ” ) located on the second floor of the building located at 300 third street, cambridge, massachusetts ( the “ building ” ). a true copy of the sublease is attached hereto as exhibit a - 1. whereas, sublandlord, as tenant, and are - ma region no. 28, llc ( successor - in - interest to three hundred third street l. l. c. ) as landlord ( “ landlord ” ) are parties to that certain lease dated april 11, 2005, as amended by that certain first amendment to lease dated july 9, 2006 and that certain second amendment to lease dated october 31, 2007 ( as amended, the “ overlease ” ). whereas, seller, sublandlord and landlord are parties to that certain consent to sublease dated as of october 23, 2006 ( the “ sublease consent ” ). a true copy of the sublease consent is attached hereto as exhibit a - 2. the sublease and the sublease consent are collectively referred to as the “ sublease documents ”. whereas, seller is in the process of completing subtenant ’ s work ( as defined in the sublease ) to prepare the subleased premises for seller ’ s use and occupancy. whereas, the subtenant ’ s work is being completed by f. l. caulfield & sons, inc. ( the “ general contractor ” ) pursuant to an agreement dated as of may 2, 2007, as modified by three prime contract change orders dated july 10, 2007, august 27, 2007 and october 1, 2007 ( as so modified, the “ construction contract ” ). a true copy of the construction contract is attached hereto as exhibit b. whereas, the subtenant ’ s work is shown on construction plans and specifications ( the “ drawings ” ) prepared by olson lewis dioli &