ERBB2 and breast cancer: HER2 overexpression or gene amplification is the only validated predictive biomarkers in use for the stratification of breast cancer patients for targeted therapies such as trastuzumab (HER2-targeted), pertuzumab (a monoclonal antibody targeted against HER2-HER3 dimerisation with other HER receptors), TDM-1 and lapatinib (against both EGFR (HER1) and HER2) [3–5].