According to the expression of specific markers, such as estrogen receptor (ER), progesterone receptor (PR), epidermal growth factor receptor 2 (HER2), and the proliferation marker Ki-67, BC has been principally classified into five subtypes: luminal A, luminal B, luminal B-like, HER2-enriched, and triple-negative breast cancer (TNBC) (3). Here, ESR1 is linked to breast cancer.