ESR1 and breast cancer: Indeed, treatment options broadly termed “endocrine therapy” targeting either estrogen production (ie, aromatase inhibitors such as anastrozole, exemestane, or letrozole) or the hormone receptor itself such as selective estrogen receptor modulators (SERMs; ie, tamoxifen) and selective estrogen receptor degraders (SERDs; ie, fulvestrant) have proven highly effective, contributing to the 39% decrease in breast cancer mortality rates observed in the UK between 1971-1973 and 2015-2017 (Cancer Research UK).