TPMT and mucositis: The frequency of GI toxicity (grade 3–4 mucositis) in this population was 8.5%; among these patients, deficiency in TPMT activity predisposed to an increased incidence of severe GI toxicity during consolidation therapy which included methotrexate (2 g/m2/week) and mercaptopurine (75 mg/m2/day, in all patients during consolidation therapy, regardless of TPMT genotype).