ERBB2 and breast cancer: The overwhelming majority of all targeted therapies approved for the treatment of breast cancer specifically target either estrogen or progesterone hormone receptors (e.g., tamoxifen, anastrozole, letrozole) or target tumors that overexpress the proto-oncogenic receptor tyrosine-protein kinase known as human epidermal growth factor receptor 2 (HER2) (e.g., trastuzumab, pertuzumab) [31].