Defective reconstitution of BM Kit⁺Flt3^high progenitors after BMT. (A) Noncompetitive chimeras were generated by lethally irradiating CD45.1 WT hosts and reconstituting them with 2.5 × 10⁵, 1 × 10⁶, or 4 × 10⁶ CD45.2 WT BM cells. Two weeks after BMT, the BM was analyzed for donor contribution to progenitor populations. Shown are representative plots from chimeras transplanted with the indicated cell number; plots are gated on donor Lin⁻ cells (or total Lin⁻ cells for the unirradiated control). (B) Shown are the mean frequencies ± SEM of Kit⁺Flt3^high cells among donor Lin⁻ cells for unirradiated controls and each of the 3 chimera cohorts 2 weeks after BMT (top left panel). Multiplying these frequencies by the total leg (tibia plus femur) BM cellularity yielded the mean numbers ± SEM of donor Lin⁻Kit⁺Flt3^high cells (top right panel). The same frequency (bottom left panel) and absolute number (bottom right panel) measurements were made on similar chimeras 4 weeks after BMT. **P < .01 compared with the corresponding unirradiated controls. N = 3 per group.