VAV1 and anemia: Mekhema null mutants died shortly after birth from a severe anemia, even though Vav1-icre expression occurs much earlier, around E11.5, in definitive HSCs (dHSCs), which are known to be involved in the generation of the definitive erythroid, myeloid, and lymphoid lineages (de Boer et al., 2003; Medvinsky et al., 2011).