ESR1 and breast cancer: The most common classification is based on its molecular features, specifically the expression of specific receptors—hormone receptors (HR; estrogen (ER) and progesterone receptor (PR)) and human epidermal growth factor receptor 2 (HER2)—and the proliferation marker Ki67, and these features are used to classify breast cancers into luminal A, luminal B, HER2-enriched, and triple-negative (TNBC)/basal-like breast cancer [15].