Breast cancer treatment has now entered a new era in which biological therapies, based on a rapidly expanding cellular and molecular understanding of breast cancer pathogenesis, have joined the standard armamentarium of surgery, radiation, chemotherapy, and hormone therapy. In 1998, the anti-HER2 humanised monoclonal antibody trastuzumab became the first biological therapy to receive US Food and Drug Administration (FDA) approval for the treatment of breast cancer, thus marking a milestone that almost certainly will be repeated with other new agents. HER2 (ErbB2) has been the focus of many therapeutic strategies because of its frequent gene amplification and overexpression in breast cancer, its role in tumourigenesis and cancer progression, and its prognostic and predictive significance in clinical studies.