Expression of IL-7Rα in bone marrow progenitors but not blood B cells. Histogram of IL-7Rα v CD19 (b and d) or control IgG1 v CD19 (a and c) in affinity-column–enriched CD34⁺ marrow cells (a and b) and peripheral blood mononuclear cells (MNC) (c and d). Both axes on all flow cytometry histograms indicate relative fluorescence intensity in a logarithmic scale (4 decades for Figs 1 and 2, and 3 decades for Figs 4 and 5). In all histograms, dashed lines indicate the fluorescence cutoff for positivity based on cells stained with control IgG. T cells were gated out on the basis of CD3, CD4, or CD8 expression. A small subpopulation (2.8%) of CD19-cells in the CD34⁺ bone marrow (b, arrow) are specifically stained by anti–IL-7Rα antibody. The proportion of cells in the four quadrants of (a) is 14.5%, 0.4%, 84.6%, and 0.5%, and of (b), 14.4%, 0.5%, 81.9%, and 3.3%. CD19⁺ (mature B) cells in peripheral blood are IL-7Rα⁻ (d), although some non-B cells in the blood show weak IL-7Rα expression. The proportion of cells in the four quadrants of (c) is 49.9%, 2.2%, 47.6%, and 0.3%, and of (d), 50.7%, 1.7%, 44.5%, and 3.1%.