Cell cycle phase of viable and apoptotic erythroblasts from conditional bcl-x^-/-mice and bled littermate controls during culture with EPO. Percentage of cells in DNA synthesis (S phase; panel A) or in Gap 0/Gap 1 (G₀/G₁) phase (panel B) at various times of culture was determined for apoptotic (TUNEL⁺) and nonapoptotic (TUNEL^-) cells. Results are mean ± SEM of 3 (controls) to 7 (conditional bcl-x^-/-) independent experiments.