ERBB2 and breast cancer: Targeted therapies for breast cancer are so far based only on the status of hormone receptors (HRs) and human epidermal growth factor receptor 2 (HER2): oestrogen and progesterone receptor-positive (ER/PR+) patients are usually treated with endocrine therapies (e.g., tamoxifen), while HER2+ patients are treated with anti-HER2 target therapies (e.g., trastuzumab) [2].