20D9-ADC was cytotoxicity to Ba/F3 cells expressing transgenic FLT3 or FLT3-ITD, to AML cell lines, and to FLT3-ITD–positive patient-derived xenograft AML cells. In vivo, 20D9-ADC treatment led to tumor burden reduction and even remission in AML xenograft models. The gene discussed is FLT3; the disease is acute myeloid leukemia.