Factor XIII is the zymogenic form of the glutaminyl-peptide g-glutamyl transferase factor XIIIa. Factor XIII is unique among transamidases in that it is a zymogen in vivo. Factor XIII is found both extracellularly in plasma and intracellularly in platelets, megakaryocytes, monocytes, placenta, uterus, liver and prostrate tissues. Description: Human serum/plasma. Alternative Names: Complement Factor XIII protein