Transglutaminase 4 also known as TGM4, TGP or hTGP, is a typical TGase that is specifically expressed in prostate tissue. TGM4 are intercellularly localizing, Ca2+-dependent enzymes, which catalyze the formation of isopeptide bonds by transferring an amine on to glutaminyl residues, thereby cross-linking glutamine residues and lysine residues in substrate proteins. TGases influence numerous biological processes including blood coagulation, epidermal differentiation, seminal fluid coagulation, fertilization, cell differentiation and apoptosis.