Defects in lymphocyte development observed in Atm^-/- mice are stem cell intrinsic. (A) At 8 and 22 weeks after BMT, the thymi of C3H mice that had received either ATM^-/- (ATM^-/- → C3H) or wild-type littermate control bone marrow cells (ATM^+/+ → C3H) were stained with anti-CD4 and anti-CD8 antibodies and were analyzed by flow cytometry. Shown is the frequency of each thymocyte subset in representative mice. (B) Eight and 22 weeks after BMT, the total number of cells in the thymi of C3H mice that had received either Atm^-/- (□) or wild-type littermate control (▪) bone marrow cells were counted, and the absolute number of each population was calculated based on the frequency of subsets as determined by flow cytometry. Shown are the combined mean and standard deviation of 3 experiments. Lethally irradiated wild-type mice reconstituted with Atm^+/+ bone marrow showed a decreased number of CD4⁺CD8⁺ double-positive and an increased number of single-positive thymocytes in comparison with untreated controls, which, based on our experience, is most likely a result of damage caused by the radiation used to condition these animals, as has been observed in previous studies.^23-25