Decreased apoptosis and remained quiescence of HSCs in the absence of Puma. (A) Detection of apoptosis of LKS cells by the TUNEL assay. LKS cells sorted from Puma^+/+ and Puma^−/− mice were treated with 4-Gy irradiation. The percentage of cells undergoing apoptosis at 24 hours after irradiation was compared with those without irradiation. Total 250 cells were calculated in each group. Percentage of apoptotic cells in Puma^+/+ and Puma^−/− was compared using the Fisher exact test. **P < .01. (B) Flow cytometric analysis of cell-cycle change of LKS cells after radiation. G₀ versus G₁ fraction in cell cycle was measured at 20 hours after 4-Gy irradiation. Representative flow cytometric profiles from 3 experiments were shown (left). The average percentages of G₀ fraction in LKS cells from 3 experiments were summarized (right). *P < .05.