Determinants of Mercaptopurine Toxicity in Paediatric Acute Lymphoblastic Leukemia Maintenance Therapy

Inclusion Criteria: * Patients have been diagnosed with Acute Lymphoblastic Leukemia * Childhood patients who were undergoing chemotherapy or continuous follow-up after completion of chemotherapy * Patients received the phase of maintenance therapy that included oral 6-MP (\>4 weeks) and completion of ≥ 6 months according to the CCLG (Chinese Children's Leukemia Group) protocol-ALL 2015 Exclusion Criteria: * Patients with high-risk ALL (presence of higher-risk features: MRD ≥ 1% at 46 day, or age \< 6 month and white blood cell (WBC) count ≥ 300×109/L with translocations t(9;22) (q34;q11) \[BCR-ABL\], t(4;11) (q21;q23) \[AF4/MLL\], t(1;19) (q23;p13) \[E2A-PBX1\] or other MLL-rearrangements) were removed