STAT5 is constitutively phosphorylated and activated in cell lines transfected with FLT3-ITD mutations, as well as leukemia cell lines and primary AML blasts containing endogenous ITD mutations, but STAT5 is not activated in cells with wild-type FLT3, despite stimulation with FL (64, 65, 69, 70). The gene discussed is FLT3; the disease is acute myeloid leukemia.