c-Myc ablation does not affect survival of developing thymocytes. (A) Annexin V staining of primary thymocytes. Thymocytes of the indicated mice were stained with antibodies against CD4 and CD8 or against lin, CD44, and CD25 followed by Annexin V and FACS analysis. Histograms of Annexin V staining are electronically gated on the indicated subsets. (B) The same subsets were also analyzed with respect to the expression of intracellular Bcl-2 levels. (C) Expression levels of Bcl-x_L and p53 mRNA in FACS-sorted DN3 and DN4 cells as determined by semiquantitative RT-PCR with 5-fold serial dilutions. Semiquantitative RT-PCR for β-actin was used as quantity control (Actin). Similar results were obtained in 3 independent experiments. (D) Protein levels of p53 in FACS-sorted cells (2 × 10⁶ per lane) as determined by Western blot.