A number of gene and chromosome alterations have been identified in sporadic breast carcinomas, and their clinical implications have been investigated. Changes in proto-oncogenes and tumor-suppressor genes, e.g., HER2, p53, and E-cadherin, and various numerical and structural chromosome alterations are strongly correlated with histological type and grade in breast carcinomas. The amount of information on these alterations has been dramatically increased by the introduction of high-throughput molecular cytogenetic approaches. In the near future, breast cancers will be classified into specific groups according to their profile of gene and chromosome alterations, allowing more effective personalized therapies targeting the associated molecular pathways.