This study aimed to investigate the frequency of drug metabolizing enzyme variants (TPMT 719A > G (*3C), ITPA 94C > A and ITPA 123G > A) and drug transporter variants (MRP4 912 C > A and MRP4 2269G > A) in Thai children with ALL and their association with 6-MP-related adverse events. Here, ABCC4 is linked to acute lymphoblastic leukemia.