Surface phenotype of IL-7Rα⁺ progenitors and CD19⁺ B-lineage cells. Affinity-column–enriched CD34⁺ marrow cells were stained with a 4-color combination. For each surface marker, paired histograms are presented: IL-7Rα v marker in the CD19⁻/T⁻ gated fraction, and CD19 v marker. The CD19⁺ fraction in CD34⁺ enriched marrow is mostly CD34⁺ pro-B cells, with a smaller population of contaminating CD34⁻/CD19⁺ mature B cells (pre–B-cell contamination is infrequent in affinity-column–enriched CD34⁺ cells). Where distinguishable, the pro–B-cell population is indicated by an arrow and the B-cell population by an arrowhead. Antigen expression in the pro–B- and B-cell CD19⁺ fractions was verified by staining with CD34 in a separate experiment. Panels include results from three different experiments (IgG controls were performed in each experiment). Each staining result was confirmed by three to eight replicate experiments on different bone marrow specimens. IL-7Rα⁺ progenitors are CD10⁺/CD45RA⁺/CD38⁺⁺/CD24⁻/CD34⁺/CD33^dim+/CD13⁺/c-kit^dim+.