Puma deficiency diminishes radiation-induced apoptosis in primitive hematopoietic cells. Analysis of apoptotic rate in LSK cell population in BM with annexin V–based apoptosis assay. BM cells were harvested from nonirradiated and irradiated (9 Gy) Puma^−/− and Puma^+/+ mice and cultured in the StemSpan serum-free expansion medium supplemented with 10 ng/mL interleukin-3 (for suppressing spontaneous apoptosis). Six hours later, BM cells were stained for LSK markers, and the apoptotic cells were identified by a fluorescence-labeled annexin V. Apoptotic LSK cells (annexin V⁺ LSK) were analyzed by flow cytometry. The percentage of annexin V⁺ LSK cells (the numbers in boxes in left panel) is significantly increased in BM cells from Puma^+/+ mice than those from Puma^−/− mice before and after radiation. The data shown are representative of flow cytometric analysis (left) and the mean of the triplicate (right). ***P < .001 (Puma^−/− vs Puma^+/+ after radiation). FACS indicates fluorescence-activated cell sorting. Bar graph data are mean ± SD.