Impaired production of TCRβ^int and TCRβ^hi thymocytes in Atm^−/− mice. (A) BrdU labeling of total thymocytes from Atm^−/− mice and age-matched Atm^+/+ controls at indicated time points after BrdU injection (histograms, left). Middle panels depict CD4 versus CD8 thymocyte profiles of Atm^−/− and Atm^+/+ mice gated on BrdU⁺ cells at days 1, 3, and 5 after injection. TCRβ profiles (right) are shown for Atm^+/+ (dotted) and Atm^−/− (solid) thymocytes gated on BrdU⁺ DP and total thymocytes. (B) Proportions of BrdU⁺ DN and DP thymocyte populations in Atm^−/− mice after pulse labeling. The graphs represent the mean percentage (±SD) of BrdU⁺ cells within the DN (left) and DP (right) thymocyte subsets from Atm^−/− mice compared with age-matched controls. Results are representative of 2 to 3 experiments per time point, with analysis of 2 to 3 animals of each genotype per experiment.