Pharmacogenetic studies have demonstrated the marked ethnic differences in the frequencies and the type of polymorphisms in TPMT, ITPA, and MRP4. This study investigated TPMT, ITPA, and MRP4 genotypes in Thai children with ALL and analyzed the association of these genotypes and the hematological toxicity that they experienced. The gene discussed is ABCC4; the disease is acute lymphoblastic leukemia.