Mutations in the ATM gene, which maps to 11q22-23, cause the multisystem recessive syndrome ataxia-telangiectasia (AT). Breast cancer has been reported in AT patients and carriers. Sporadic breast cancer is associated with loss of heterozygosity at or in the region of ATM and chromosomal abnormalities involving 11q23. We have investigated the chromosomes, nuclei and released chromatin fibers from nine primary breast carcinoma and eight cell lines by fluorescence in situ hybridization with four fluorochrome-labeled cosmids spanning the ATM gene. The ATM gene was disrupted in one primary breast carcinoma and in the cell lines MDA-MB-231 and MCF-7. The role of these aberrations in breast carcinomas, which may lead to gene dosage or dominant negative effects on gene function, requires further investigation.