Fibrinogen is an abundant plasma protein (5-10uM) produced in the liver. The intact protein has a MW of 340kD. It is composed of 3 pairs of disulfide-bound polypeptide chains named Aalpha, Bbeta and gamma. Fibrinogen is a triglobular protein consisting of a central E domain and terminal D domains. Proteolysis by thrombin results in release of Fibrinopeptide A (FPA, Aalpha1-16) followed by Fibrinopeptide B (FPB, Bbeta1-14) and the fibrin monomers that result polymerize in a half-overlap fashion to form insoluble fibrin fibrils. The chains of fibrin are referred to as alpha, beta and gamma, due to the removal of FPA and FPB. The polymerised fibrin is subsequently stabilized by the transglutaminase activated Factor XIII that forms amide linkages between gamma chains and, to a lesser extent, alpha chains of the fibrin molecules. Proteolysis of fibrinogen by plasmin initially liberates C-terminal residues from the Aalpha chain to produce fragment X (intact D-E-D, which is still clottable).