definiens: means that certain Researc h, Development and Commercialization Agreement between Counterparty and the Seller dated May 24, 2 004, as amended by Amendment No . 1 thereto dated January 6, 2006, Amendment No . 2 thereto dated as o f January 1, 2006, a letter agreement styled as Ame ndment No . 3 thereto dated November 20, 2006, a letter agr eement styled as Amendment No . 4 thereto dated Augu st 20, 2007, Amendment No . 5 thereto dated as of April 1, 2011 and Amendment No . 6 thereto dated March 29, 20 12, together with that certain letter agreement dated A pril 18, 2013 from Counterparty to the Seller and t hat certain letter agreement dated October 17, 2014 from Counte rparty to the Seller