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Background: The objective of this work was to demonstrate that autoantibodies in breast cancer sera are not epiphenomena, and exhibit unique immunologic features resembling the rheumatic autoimmune diseases., Methods: We performed a comprehensive study of autoantibodies on a collection of sera from women with breast cancer or benign breast disease, undergoing annual screening mammography. All women in this study had suspicious mammography assessment and underwent a breast biopsy. We used indirect immunofluorescence, the crithidia assay for anti-dsDNA antibodies, and multiple ELISAs for extractable nuclear antigens., Results: Autoantibodies were detected in virtually all patients with breast cancer, predominantly of the IgG1 and IgG3 isotypes. The profile detected in breast cancer sera showed distinctive features, such as antibodies targeting mitochondria, centrosomes, centromeres, nucleoli, cytoskeleton, and multiple nuclear dots. The majority of sera showing anti-mitochondrial antibodies did not react with the M2 component of pyruvate dehydrogenase, characteristic of primary biliary cirrhosis. Anti-centromere antibodies were mainly anti-CENP-B. ELISAs for extractable nuclear antigens and the assays for dsDNA were negative., Conclusions: The distinctive autoantibody profile detected in BC sera is the expression of tumor immunogenicity. Although some of these features resemble those in the rheumatic autoimmune diseases and primary biliary cirrhosis, the data suggest the involvement of an entirely different set of epithelial antigens in breast cancer. High titer autoantibodies targeting centrosomes, centromeres, and mitochondria were detected in a small group of healthy women with suspicious mammography assessment and no cancer by biopsy; this suggests that the process triggering autoantibody formation starts in the pre-malignant phase and that future studies using validated autoantibody panels may allow detection of breast cancer risk in asymptomatic women. Autoantibodies developing in breast cancer are not epiphenomena, but likely reflect an antigen-driven autoimmune response triggered by epitopes developing in the mammary gland during breast carcinogenesis. Our results support the validity of the multiple studies reporting association of autoantibodies with breast cancer. Results further suggest significant promise for the development of panels of breast cancer-specific, premalignant-phase autoantibodies, as well as studies on the autoantibody response to tumor associated antigens in the pathogenesis of cancer.