ESR1 and breast carcinoma: The most commonly used endocrine therapeutic agents that target ERα-positive breast cancers include ER modulators (e.g., tamoxifen, which selectively antagonizes ERα function), ER downregulators (e.g., fulvestrant, also known as ICI 182,780 and faslodex, which selectively downregulates ERα), and aromatase inhibitors (e.g., letrozole and anastrozole, which repress estrogen production by attenuating aromatase activity)3,5.