ERBB2 and breast cancer: Based on the results of early clinical trials using the first anti-HER2 targeted agent, trastuzumab, HER2-positive breast cancer has been defined as tumors with an immunohistochemistry (IHC) score of 3 + for HER2 staining or IHC score of 2 + with HER2 gene amplification by in situ hybridization (ISH) assay [4–6].