ESR1 and breast cancer: ER+ breast cancers are effectively treated with three types of endocrine therapies: (i) selective estrogen receptor modulators (SERMs), which block estrogen access to the estrogen receptor, (ii) selective estrogen receptor downregulators (SERDS), which downregulate the estrogen receptor, or (iii) aromatase inhibitors (AIs), which inhibit the production of estrogen [16–18].