c-Myc ablation inhibits proliferation of DN4-stage thymocytes. (A) 7AAD staining of permeabilized thymocytes. Thymocytes from the indicated mice were surface stained with anti-lin antibodies as well as anti-CD44 and anti-CD25, followed by staining with 7AAD and FACS analysis. Histograms are electronically gated lin^-/CD44^-/CD25⁺ (DN3) or lin^-/CD44^-/CD25^- (DN4) cells. Percentages in histograms represent cells in S/G₂/M phases of the cell cycle. Histogram bars represent cumulative measurements of 8 control and 7 LckCre-Myc^fl/fl cycling DN4 cells. Error bars indicate SD. (B) Cell size. Forward scatter (FSC) profiles of the indicated mice and subsets are shown. (C) Semiquantitative RT-PCR for pTα and TCRβ mRNA expression in DN3 and DN4 thymocytes. RT-PCR for β-actin is used as quantity control. Similar results were observed in 3 independent experiments. (D) Intracellular TCRβ expression in DN3 and DN4 thymocytes. Cells were surface stained as in panel A followed by permeabilization and staining with anti-TCRβ antibodies and FACS analysis. Similar results were obtained in more than 5 independent experiments.