TGF-β, but not other TGF-β family members, confers a cytostatic effect on HSCs ex vivo. Freshly isolated CD34⁻KSL HSCs were sorted clonally into 96-well microtiter plates and incubated in the presence of SCF, TPO, and TGF-β1; SCF, TPO, and Activin-A; or SCF, TPO, and Nodal. At day 2 of culture, nondivided cells that stayed dormant were assessed under an inverted microscope.