Studies in several populations, mainly inflammatory bowel disease (IBD), have shown that TPMT variant alleles and lower TPMT activity are associated with a higher risk of bone marrow toxicity.[7,11–13] Patients carrying two variant TPMT alleles are especially at risk for severe myelotoxicity[11] and require either a 10-fold lower dose or alternative therapy (e.g. methotrexate).[6,8,14] For patients with intermediate TPMT activity carrying one variant allele, more controversy exists. The gene discussed is TPMT; the disease is inflammatory bowel disease.