Summary
         
         Fibrinogen Matsumoto I is a novel hereditary dysfibrinogen identified in a 1-year-old
            boy with Down’s syndrome. Though he showed no apparent bleeding or thrombotic tendency,
            he had a congenital heart disease. Preoperative coagulation tests of his plasma revealed
            a prolonged thrombin time and the fibrinogen level determined by the thrombin time
            method was markedly decreased. Molecular weight of fibrinogen chains showed apparently
            normal Aα, Bβ-, and γ-chains. The rate of fibrinopeptide release was normal, whereas
            fibrin polymerization was delayed. Fibrinogen γ-chain gene fragments from the propositus
            were amplified by polymerase chain reaction then sequenced. The triplet GAT, coding
            for the amino acid residue γ364, was replaced by CAT, resulting in the substitution
            of Asp→His. This residue is adjacent to the Tyr-363 that is demonstrated to be the
            primary site for fibrin polymerization. Our results indicate that the residue γ364
            Asp is essential for normal polymerization of fibrin monomer.