Phenotypic analyses of B-ALL arising from p19^ARF-null HSC versus pro-B cells. Primary recipients injected with p210BCR/ABL-expressing p19^ARF-null HSC or pro-B cells were killed approximately 2 to 3 weeks after transplantation. Leukocytes from the recipients' bone marrow were harvested for flow cytometric phenotypic analysis. (A) Cells derived from HSC (left) contained approximately 3-fold more (2.1% vs 0.7%) mature B cells (IgM⁺) as cells derived from pro-B stage (right). The plots shown were gated on EGFP⁺ lymphocytes. (B) EGFP⁺ B-ALL cells (B220⁺, CD19⁺, IgM⁻) derived from HSCs (shaded in gray) showed significantly higher CD49e (VLA5) expression than the cells arising from pro-B (white area). (C) 10⁶ flow-sorted EGFP⁺ B-ALL cells from bone marrow of both HSC and pro-B recipients were reverse-transcribed to cDNA and analyzed by real-time quantitative PCR. Results are shown for 3 independent experiments, with each bar representing the average of triplicates. Error bars are standard deviation. Each gene expression value is shown relative to expression of hypoxanthine-guanine phosphoribosyl transferase (HPRT). Significance was determined with Student paired, 1-tailed t test (*P < .05; **P < .01; ***P < .005; ****P < .001).