TPMT is involved in the metabolismof thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine,which are cytotoxic immunosuppressant compounds used to treat childhoodacute lymphoblastic leukemia, inflammatory bowel disease, and rheumatologicaldiseases.89−91 Their methylation by TPMT reduces their cytotoxiceffects.92,93 Surprisingly, FMeTeSAM is as good a methylatingagent under saturating conditions as SAM, and the Km for FMeTeSAM is nearly the same as that for SAM. Here, TPMT is linked to inflammatory bowel disease.