We performed preclinical drug testing of the BCL-2 inhibitor, venetoclax, and the FLT3 inhibitor, gilteritinib, in two FLT3-mutant BCL11B-a PDX models: one T/myeloid MPAL and one acute undifferentiated leukemia (AUL)…Single agent gilteritinib showed increased efficacy compared with venetoclax; however, gilteritinib was unable to prevent continued leukemic growth over the 8-week treatment. In contrast, combination therapy rapidly and stably reduced leukemic burden to pre-treatment levels....We showed that the combination of FLT3 and BCL-2 inhibition is highly effective at reducing leukemia burden in two preclinical models of BCL11B-a lineage ambiguous leukemia.