c-Myb is required for normal expression of IL-7Rα on lymphoid progenitors. (A) Flow cytometry of IL-7Rα surface expression on pro-B cells from 3- to 4-week-old mice. The upper panel is gated on B220⁺CD19⁺c-kit⁺ cells; the lower panel is gated on B220⁺c-kit⁺ cells. Plots are representative of 3 to 9 mice per genotype. (B) Flow cytometry of splenocytes from c-Myb^+/+ and c-Myb^Plt4/Plt4 mice. Numbers adjacent to the boxes indicate percentages of CD4⁺ and CD8⁺ T cells among all live cells. Dot plots are representative of 3 mice per genotype. Note that the relative increase in CD8⁺ and CD4⁺ cells is due to the loss of B cells in c-Myb^Plt4/Plt4 mice. (C) Flow cytometry of IL-7Rα surface expression on splenic T cells from c-Myb^+/+ and c-Myb^Plt4/Plt4 mice, gated for CD8⁺ or CD4⁺ cells as indicated. Plots are representative of 3 mice per genotype. (D) Flow cytometry of density-fractionated and lineage-depleted bone marrow of 5- to 9-week-old c-Myb^+/+ and c-Myb^Plt4/Plt4 mice on a Rag1^gfp/+ background. Cells at the CLP stage of development were identified as Sca-1⁺c-kit^int GFP⁺; numbers in plots indicate the percentage of cells in each gated area. (E) Flow cytometry of IL-7Rα surface expression on Sca-1⁺c-kit^int GFP⁺ cells. Plots are representative of 4 to 5 mice per genotype.