High responsiveness of FA IL-3Rα–positive blasts to IL-3. (A) FA IL-3Rα–positive blasts displayed a significantly higher proliferative rate in the presence of IL-3. Human CD45⁺IL-3Rα⁺ cells from the bone marrow of FA leukemic mice were cultured in the presence of IL-3 (10 ng/mL) for 15 minutes, followed by BrdU incorporation assay. (B) Cells described in panel A were continuously cultured in the presence of 10 ng/mL of IL-3 for 2 days, and then shifted to IL-3 free medium for an additional 2 days. Cell apoptosis was then determined by annexin V/7AAD double staining, followed by flow cytometric analysis. (C-D) Quantitative analysis of BrdU incorporation and apoptosis. Data are presented as the percentage of BrdU incorporated or of apoptotic cells. Results of individual samples were plotted after normalization with total cell number. Results are means ± SD of 3 independent experiments.