Schematic representation of the metabolism of mercaptopurine and the enzymes involved. GDP, guanosine diphosphate; GMP, guanosine monophosphate; GMPS, guanosine monophosphate synthase; GTP, guanosine triphosphate; HPRT, hypoxanthine phosphoribosyltransferase; IMPDH, inosine monophosphate dehdrogenase; IDP, inosine diphosphate; IMP, inosine monophosphate; ITP, inosine triphosphate; ITPA, inosine triphosphate pyrophosphatase; K, kinase; TPMT, thiopurine S-methyltransferase; XO, xanthine oxidase.