The etiology of breast cancer involves a complex interplay of various factors, including genetic alterations. Many studies have been devoted to the identification and characterization of mutations that occur frequently during breast tumorigenesis. The major types of genetic abnormalities that are frequently observed in breast tumors are amplification of protooncogenes (MYC, ERBB2) and DNA from chromosome band 11q13; mutation of TP53; and loss of heterozygosity from chromosomes and chromosome arms 1, 3p, 6q, 7q, 8p, 11, 13q, 16q, 17, 18q, and 22q. The latter may correspond to losses or inactivations of tumor suppressor genes. Recently, linkage analyses of large families with a predisposition to breast cancer have been performed in order to map breast cancer susceptibility genes (TP53, BRCA1, BRCA2). The findings have thrown light on the molecular mechanisms of breast cancer and have enabled various genetic markers to be used in clinical oncology.