ESR1 and breast cancer: At the molecular level, BC is mainly classified into four different subtypes, luminal A, luminal B, HER2-enriched, and triple-negative breast cancer (TNBC), based on the expression of epidermal growth factor receptor 2 (HER2), estrogen receptor alpha (ER), and progesterone receptor (PR) (Dai et al., 2015; Prat et al., 2015; Azamjah et al., 2019).