The enzyme TPMT competes with the formation of 6TGN, as it methylates 6MP and thus create relatively non-toxic metabolites. TPMT heterozygous patients with one wild type and one low-activity allele have a higher risk of myelosuppression and treatment interruption compared to patients with TPMT wild type. Furthermore, TPMT heterozygous patients have a reduced risk of relapse and a higher risk of secondary malignancy compared to patients with TPMT wild type.