Coagulation factor XIII is the last enzyme produced in the blood coagulation cascade. The plasma form of this protein is a hetero-tetramer consisting of two A and two B subunits. Intracellular FXIII, meanwhile, is a homodimer comprising of two A subunits only. Catalytic function is carried out only by the A subunits, and involves acting as a transglutaminase to cross-link fibrin molecules via a gamma-glutamyl-epsilon-lysine link, thereby stabilizing the fibrin clot. Additionally, FXIII also cross-links several other protein substrates in the plasma and subendothelium, including bronectin, von Willebrand factor, vitronectin, collagen, coagulation factor V, thrombospondin, and plasminogen activator inhibitor type 1.