The function of TPMT (thiopurine S-methyltransferase, Ensembl gene identifier ENSG00000137364) is as follows. Catalyzes the S-methylation of thiopurine drugs such as 6- mercaptopurine (also called mercaptopurine, 6-MP or its brand name Purinethol) and 6-thioguanine (also called tioguanine or 6-TG) using S- adenosyl-L-methionine as the methyl donor (PubMed:18484748, PubMed:657528). TPMT activity modulates the cytotoxic effects of thiopurine prodrugs. A natural substrate for this enzyme has yet to be identified. {ECO:0000269|PubMed:18484748, ECO:0000269|PubMed:657528, ECO:0000305}.