TPMT is an enzyme that metabolizes thiopurine drugs via S-adenosyl-L-methionine as the S-methyl donor and S-adenosyl-L-homocysteine as a byproduct. Thiopurine drugs such as 6-mercaptopurine are used as chemotherapeutic agents. Genetic polymorphisms that affect this enzymatic activity are correlated with variations in sensitivity and toxicity to such drugs within individuals. A pseudogene for this locus is located on chromosome 18q.
[1] "Human thiopurine methyltransferase: molecular cloning and expression of T84 colon carcinoma cell cDNA."Honchel R., Aksoy I.A., Szumlanski C., Wood T.C., Otterness D.M., Wieben E.D., Weinshilboum R.M.Mol. Pharmacol. 43:878-887(1993) [2]