PBX1 and myeloid leukemia: These findings are corroborated in our in vitro FLP model: the lymphoid potential of lin- FLPs expressing E2A-PBX1 is blocked prior to the CLP stage (Fig 5), these FLPs are amenable to myeloid differentiation induced by GM-CSF, and they confer myeloid leukemia upon transplanted mice (Fig 4).