ATM and breast cancer: Many clinical studies have tested the hypothesis that ATM heterozygote breast cancer patients exhibit enhanced radiosensitivity; yet, most of those studies failed to show a clear association between ATM mutation status and increased radiosensitivity (Ramsay et al. 1996; Appleby et al. 1997; Ramsay et al. 1998; Weissberg et al. 1998; Shayeghi et al. 1998).