Loss of p57 results in a proliferative defect, induction of apoptosis, and excessive phosphorylation of Rb in thymocytes. (A) Representative flow cytometric histograms for BrdU staining in DN3 and DN4 thymocytes from Lck-Cre/p57^[+]/+ and Lck-Cre/p57^[+]/F mice injected with BrdU at 8 weeks of age. (B) Determination of the proportion of cells positive for BrdU as in panel A. Data are means ± SD for 4 mice. *P < .05, ***P < .005. (C) Representative histograms for annexin V staining in DN3 and DN4 thymocytes from Lck-Cre/p57^[+]/+ and Lck-Cre/p57^[+]/F mice at 8 weeks of age. (D) Determination of the proportion of cells positive for annexin V staining as in panel C. Data are means ± SD for 4 mice. ***P < .005. (E) IB analysis of phosphorylated Rb in DN3 thymocytes from Lck-Cre/p57^[+]/+ and Lck-Cre/p57^[+]/F mice at 8 weeks of age. HSP70 was examined as a loading control. (F-G) RT and real-time PCR analysis of Mcm3, Cdc6, and Cdt1 mRNAs (F) as well as of ATM and ASPP1 mRNAs (G) in DN3 thymocytes from Lck-Cre/p57^[+]/+ and Lck-Cre/p57^[+]/F 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.