TPMT and inflammatory bowel disease: It was initially suggested that unusually high activity of TPMT may shift the metabolic machinery toward thiopurine methylation; this has been reported to result in low 6-TGN levels and unfavorable clinical response in IBD and acute lymphoblastic leukemia patients (Lennard et al., 1990; Ansari et al., 2002; Bloomfeld and Onken, 2003; Cuffari et al., 2004).