Alterations in the p53 gene are found in 20% to 40% of breast cancers and are generally associated with factors of adverse prognostic significance. In most instances, point mutations modify the confirmation of p53, causing the gene to accumulate in the nuclei of tumor cells. These alterations can be detected via molecular analysis or immunohistochemical methods. More recent studies have demonstrated that accumulation of the p53 protein in tumor cells may induce an immune response with presence of anti-p53 antibodies in the serum of cancer patients. Assaying serum anti-p53 antibody is a new approach to investigation of the status of the p53 gene in a tumor.