Cell cycle analysis of primitive hematopoietic stem cells after stimulation. Lin⁻Sca-1⁺ BM cells from Mx–c-fos or control littermates were cultured with SCF, IL-3, and IL-6 in the presence (200 U/mL) or absence of IFN-/β. (A) After 24 hours, the cells were lysed and the nuclei were stained with propidium iodide. DNA content in the nuclei was determined by FACS. Percentage of PI-labeled nuclei in S/G2/M phase of the cell cycle is indicated. (B) After 36 hours, cells were pulsed with BrdU for 3 hours. Incorporated BrdU was detected by anti-BrdU monoclonal antibody and positive cells were counted. Results represent mean and SD of four dishes. The data presented are representative of three independent experiments.