Additional ablation of E2F1 attenuates the defects of immature and mature T cells in p57-deficient mice. (A) Gross appearance of the thymus of Lck-Cre/p57^[+]/+, Lck-Cre/p57^[+]/F, and Lck-Cre/p57^[+]/F/E2f1^−/− mice at 8 weeks of age. Scale bar, 2 mm. (B) Total number of thymocytes for individual Lck-Cre/p57^[+]/+ (n = 7), Lck-Cre/p57^[+]/F (n = 7), and Lck-Cre/p57^[+]/F/E2f1^−/− (n = 5) mice at 8 weeks of age. ***P < .005. (C-D) Representative flow cytometric analysis of CD4 vs CD8 on thymocytes (C) as well as of CD44 vs CD25 on electronically gated lineage-negative DN thymocytes (D) from Lck-Cre/p57^[+]/+, Lck-Cre/p57^[+]/F, and Lck-Cre/p57^[+]/F/E2f1^−/− mice at 8 weeks of age. Percentages of each fraction are indicated. (E) Absolute cell number for thymocyte subsets from individual Lck-Cre/p57^[+]/+ (n = 7), Lck-Cre/p57^[+]/F (n = 7), and Lck-Cre/p57^[+]/F/E2f1^−/− (n = 5) mice at 8 weeks of age determined by flow cytometry. **P < .01, ***P < .005. (F) Number of TCRβ-positive cells among splenocytes from individual Lck-Cre/p57^[+]/+ (n = 7), Lck-Cre/p57^[+]/F (n = 7), and Lck-Cre/p57^[+]/F/E2f1^−/− (n = 5) mice at 8 weeks of age. (G-H) Splenic CD3⁺ T cells from mice of the indicated genotypes at 8 weeks of age were stimulated with anti-CD3ε (5 μg/mL) for 36 hours and exposed to BrdU during the final 1 hour of incubation. They were then stained with anti-BrdU and propidium iodide, and the percentages of BrdU-positive cells (G) and of sub-G₁ (apoptotic) cells (H) were determined by flow cytometry. Data are means ± SD for 3 mice.