Expression of stabilized β-catenin induces p53-independent DNA damage and OIS and p53-dependent apoptosis in thymocytes. (A) Total number of thymocytes in p53−/− CAT-Tg mice at different ages. The first two bars (light blue) represent the initial decrease in thymic cellularity in young mice, and the right bar (dark blue) indicates regained thymic cellularity after 66 days of age. (B) Immunofluorescence staining of γH2AX in fixed DN cells from C57BL/6, p53−/−, and p53−/− CAT-Tg mice. (C) SA β-galactosidase expression in cryosections from a 50-day-old lymphoma-free p53−/− CAT-Tg mouse with a small thymus. (D) Annexin V staining of electronically gated DN4 thymocytes from C57BL/6, CAT-Tg, and lymphoma-free p53−/− CAT-Tg mice (≤65 days old). The data are averages of data from more than three mice for each genotype in one experiment, which is representative of four experiments. (E) BrdU incorporation in electronically gated DN thymocytes from young and lymphoma-free p53−/− CAT-Tg mice (growth arrested) and relatively old lymphoma-bearing p53−/− CAT-Tg mice (proliferating). The graph shows the average of data from more than two mice for each genotype in one experiment and is representative of two experiments. (F) RT-PCR analysis of the expression of cell cycle regulators in sorted DN thymocytes from young lymphoma-free p53−/− CAT-Tg mice (growth arrested) (n = 3) and old lymphoma-bearing p53−/− CAT-Tg mice (proliferating) (n = 3).