Model for the regulation of repopulative capacity by the expression level of Smad4. The precise level of Smad4 can modulate the sensitivity to TGF-β and affect the repopulative capacity of HSPCs in vivo. Smad4-deficient cells have decreased repopulative capacity and are insensitive to TGF-β. Overexpression of Smad4 leads to increased sensitivity to TGF-β, which translates into reduced repopulative capacity.