ESR1 and breast cancer: Currently, available molecular targeted therapy for BC has achieved some great success including estrogen receptor (ER)-targeting agents (e.g., tamoxifen) and human epidermal growth factor receptor 2 (HER2)-targeting therapeutics (e.g., trastuzumab) (Munagala et al., 2011; Liedtke and Kiesel, 2012; Miller, 2014).