Breast cancer is the most common cancer in Western women, with a lifetime risk of 1/10. Over the last decade, our knowledge of the molecular events involved in breast cancer biology and pathology has vastly improved. Breast cancer is caused by the activation or inactivation of numerous types of genes. The sequence of gene modifications in tumour progression is unclear, and it differs significantly from the greatest example of tumour progression now available, colo-rectal carcinoma. Despite this, the vast number of genetic changes found in breast tumours fits the multistep carcinogenesis paradigm. Breast