This family consists of thiopurine S-methyltransferase proteins from both eukaryotes and prokaryotes. Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyses S-methylation of aromatic and heterocyclic sulfhydryl compounds, including anticancer and immunosuppressive thiopurines [1].

1: Functional characterization of the human thiopurine S-methyltransferase (TPMT) gene promoter. Fessing MY, Krynetski EY, Zambetti GP, Evans WE; Eur J Biochem 1998;256:510-517. PMID:9780226