Flow chart of patient inclusion or exclusion. Patients (n = 545) participated in the randomized MTX/6MP maintenance therapy study¹⁶  or they developed SMNs and had their data on maintenance therapy 6MP/MTX dosage and blood counts retrieved. Data on both 6MP/MTX doses and blood counts were not available for all the 545 patients. TPMT phenotyping (n = 524) or genotyping or both were available for 609 patients.