BCL-2 overexpression fails to rescue the generation of TCRβ^int thymocytes in ATM-deficient mice. BrdU labeling of total thymocytes from Atm^−/− and Atm^+/+ mice segregating the Bcl2 transgene at indicated time points after BrdU injection (histograms, left). CD4 versus CD8 thymocyte profiles (left) gated on BrdU⁺ cells are displayed at 1 or 3 days after injection in Atm^−/− and Atm^+/+ mice segregating the Bcl2 transgene. TCRβ profiles (right) gated on BrdU⁺ cells are shown for DP thymocytes from Atm^−/− (solid) and Atm^+/+ (dotted) mice segregating the Bcl2 transgene, 3 days after BrdU injection. Results are representative of 2 experiments per time point, with 2 to 3 animals of each genotype.