ESR1 and breast cancer: Based on the heterogeneity of gene expression profiles (i.e., estrogen receptor [ER], progesterone receptor [PR], and human epidermal growth factor receptor 2 [HER2]), BC is typically classified into four main molecular subtypes: luminal A (ER and/or PR-positive, HER2-negative), luminal B (ER and/or PR-positive, HER2-positive), HER2-enriched (ER and PR-negative, HER2-positive), and triple-negative (ER, PR, and HER2-negative) (Carey et al., 2006).