ESR1 and breast cancer: Breast cancer (BC) is the most prevalent cancer among women and the second leading cause of cancer-related mortality worldwide.[1] BC is categorized into 3 major subtypes based on molecular markers: estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2): hormone receptor-positive, HER2-positive, and triple-negative breast cancer (TNBC).