ESR1 and breast cancer: The treatment of breast cancer varies based on molecular classification and subtype, defined by the presence of ERs, PRs, and human epidermal growth factor receptor 2 (HER2) status: hormone receptor-positive with luminal A (ER+PR+HER2−) and luminal B (ER+PR+HER2+) phenotypes, HER2-positive (ER−PR−HER2+), and triple-negative/basal-like (ER−PR−HER2−).