ERBB2 and breast cancer: Overexpression of HER2 has been documented in approximately 18–23% of human breast cancers8 and is considered a poor prognostic marker.9 Clinical Phase III studies of trastuzumab (Herceptin®; Genentech, San Francisco, CA, USA) in breast cancer have shown a great benefit, particularly for patients with tumours with HER2 gene (HER2) amplification.10–12 Current recommendations of the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) include determination of HER2 status in all invasive breast cancers using immunohistochemistry or ISH.8