Since tamoxifen was first discovered for its antitumor function in BC, several anti-estrogen regimens, including SERMs (selective estrogen receptor modulators), SERDs (selective estrogen receptor downregulators), AIs (aromatase inhibitors) and GnRHa (gonadotropin-releasing hormone antagonists), have been developed to improve patient outcomes [3], yet endocrine resistance and disease progression still occurs in approximately 50% of these patients [4]. Here, ESR1 is linked to breast cancer.