Most ER+ breast cancer patients initially responded well to endocrine therapy that attenuates ERα signaling, either by blocking the production of estrogens via aromatase inhibitors or antagonizing the activity of estrogens by competitive binding of ER antagonists such as tamoxifen and fulvestrant, a SERD [62,63,64,65,66,67]. The gene discussed is ESR1; the disease is breast carcinoma.