ESR1 and breast cancer: Pioneered by the AR (Androgen receptor) degrader ARV-110 (NCT03888612) and ER (Estrogen receptor) degrader ARV-471 (NCT04072952), developed by Arvinas Inc. for the treatment of prostate cancer and breast cancer, respectively, the field has seen at least 15 degraders in a clinical trial (Table 1) (Mullard 2021).