CYP3A4 and acute lymphoblastic leukemia: <b>Background/Objectives</b>: Mechanisms underlying treatment resistance in hematopoietic malignancies such as acute lymphoblastic leukemia (ALL) include (1) enhanced activity of anticancer drug efflux mechanisms (MRP1); (2) suppressed activity of anticancer drug influx mechanisms (ENT-1); (3) enhanced drug detoxification activity (AKR1B10, AKR1C3, CYP3A4); (4) influence of the tumor microenvironment (GRP94), etc. We conducted this study to comprehensively and clinically examine treatment resistance due primarily to a decrease in the tumor intracellular anticancer drug concentrations.