One possible explanation for this difference is that in MV4-11 and other Flt3-ITD+ AML cells, Flt3-ITD activates not only Fes, but also Syk [30] and several myeloid members of the Src kinase family (Hck, Lyn, and Fgr) [31–33]. Here, HCK is linked to acute myeloid leukemia.