ERBB2 and breast cancer: Overexpression of human epidermal growth factor receptor 2 (HER2/ErbB2) in breast cancer is a poor prognostic indicator, as these tumors tend to be more aggressive and associated with higher recurrence rates.46 Trastuzumab (Herceptin), a humanized anti-HER2 monoclonal antibody targeting the extracellular domain of this receptor, has been shown in both the metastatic47 and the adjuvant48 setting to dramatically change the survival in HER2 positive breast cancer.