Model of Cbfb-MYH11–induced aberrant differentiation and leukemia initiation. We propose that expression of Cbfb-MYH11 in HSCs or early progenitors causes an abnormal and incomplete differentiation that culminates in a Csf2rb⁺ cell population. Our results indicate that by the time the Cbfb-MYH11–expressing cells have reached the Csf2rb⁺ stage, they are no longer capable of initiating leukemia.