Atm^-/-mice are more sensitive to conditioning than wild-type littermate controls.Atm^-/- and Atm^+/+ mice were conditioned with cyclophosphamide, anti-CD4, and anti-CD8 monoclonal antibodies and were injected with 10⁸ C3H bone marrow cells. Sixteen weeks after BMT, PBMCs were analyzed for the presence of C3H-derived H-2K^k- or host-derived H-2^b-positive cells by flow cytometry. (A) Shown are representative examples of mice 16 weeks after BMT from 1 of 3 independent experiments. Wild-type Atm^+/+ mice did not show the presence of donor-derived cells in PBMCs 16 weeks after transplantation. Although most (7 of 9) Atm^-/- animals became fully chimeric with more than 99% donor-derived PBMCs (Chimeric), a few (2 of 9) showed no donor-derived cells (Nonchimeric). (B) Shown are representative examples of mice 52 weeks after BMT. Note that none of the Atm^-/- mice that failed to become chimeric survived to 52 weeks.