The endocrine-therapy agents that are currently used for ERα-positive breast cancer include fulvestrant (also known as ICI 182,780 and faslodex, the ER downregulator that selectively downregulates and/or degrades ERα), tamoxifen (the ER modulator that selectively antagonizes ERα function), and aromatase inhibitors (e.g. letrozole and anastrozole, which inhibit estrogen production by attenuating aromatase activity) [17, 19]. Here, ESR1 is linked to breast cancer.