ESR1 and breast cancer: The therapeutic strategies of ER+ BC include, on one hand, the reduction of circulating ovarian oestrogens or of peripherally produced oestrogen (in postmenopausal women) with aromatase inhibitors, and on the other hand, the application of selective ER modulators for receptor blockade by tamoxifen, which is a selective ER modulator having both ER agonist and antagonist properties, working primarily as antagonists in tumors, or by fulvestrant, which is a selective ER down-regulator and a pure ER antagonist that accelerates the degradation of the ER.