My-bi HSCs generate reduced levels of thymic precursors. Data are from an analysis of 2 independent My-bi HSC clones. BMCs derived from My-bi HSC–repopulated mice were mixed with equal numbers of control BMCs. Control BMCs served as an internal standard. The cell mixture was injected together with an excess of host-type BMCs. The number of mice that showed donor-type cells in thymus derived from the My-bi HSCs (▪) or the control BM (□) were enumerated by immunofluorescence. CFUt frequencies were calculated from the number of mice that had no thymic donor-type cells at 4 weeks after injection of mixtures of BMCs from My-bi HSC–repopulated and control mice. For details, see “Materials and methods.”