Background: Acute lymphoblastic leukemia (ALL) is responsible for nearly one third of all childhood cancers and can be cured with combination chemotherapy alone. 6- Mercaptopurine (6-MP) is one of the most commonly prescribed chemotherapeutic agents to treat ALL. The main toxicity of this 6-MP is myelosuppression that usually results in interruption or even discontinuation of effective anticancer therapy, contributing to an increased incidence of late relapse.6-MP intolerance is commonly associated with a deficiency in the activity of the enzymes thiopurine S-methyltransferase (TPMT) and NUDT15 (Nucleoside diphosphate-linked moiety X motif 15) that metabolize 6-MP. Objectives: 1. To determine the pattern of TPMT and NUDT15 gene polymorphism in patients of acute lymphoblastic leukemia 2. To estimate the serum concentration of 6MMP in patients on maintenance therapy of 6 MP at 24 weeks/ during maintenance therapy 3. To find the association between gene polymorphism with the myelosuppression due to 6 MP in ALL patients (treatment related toxicity)