Molecular genetic analysis of breast cancers indicates that the mechanisms underlying tumorigenesis are complicated. Many oncogenes and tumour suppressor genes have been implicated, encoding proteins that are important at many levels of cell regulation, from cell surface molecules responding to external signals (eg ERBB2) to nuclear factors controlling gene transcription (eg TP53, MYC). Several correlations have been found between certain genetic events and clinical outcome and have therefore proved useful prognostic indicators. The mapping and cloning of genes important in familial breast cancers (eg BRCA1) have provided the essential tools for pinpointing the genes that may be critical in early stage breast cancer as well as for developing genetic tests for predicting carrier status in breast cancer families. Clarification of the molecular consequences of mutation in breast cancer associated genes is beginning to address the factors that drive a normal breast cell to change into a breast cancer cell. However, these studies are still in their infancy, and considerable research will be required to complete the picture.