The pre-T cell differentiation block in p57-deficient mice is overcome by additional ablation of p53. (A) RT and real-time PCR analysis of p21, Noxa, and Bax mRNAs in DN3 thymocytes from Lck-Cre/p57^[+]/+, Lck-Cre/p57^[+]/F, and Lck-Cre/p57^[+]/F/E2f1^−/− mice at 8 weeks of age. Normalized data are expressed relative to the corresponding value for control mice and are means ± SD for 3 mice. ***P < .005. (B) Gross appearance of the thymus of Lck-Cre/p57^[+]/+, Lck-Cre/p57^[+]/F, Lck-Cre/p57^[+]/+/p53^−/−, and Lck-Cre/p57^[+]/F/p53^−/− mice at 7 weeks of age. Scale bar, 2 mm. (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, Lck-Cre/p57^[+]/+/p53^−/−, and Lck-Cre/p57^[+]/F/p53^−/− mice at 7 weeks of age. Percentages of each fraction are indicated. (E) Absolute cell numbers determined by flow cytometry for thymocyte subsets from individual Lck-Cre/p57^[+]/+, Lck-Cre/p57^[+]/F, Lck-Cre/p57^[+]/+/p53^−/−, and Lck-Cre/p57^[+]/F/p53^−/− mice at 7 weeks of age (n = 3). ***P < .005.