ESR1 and neoplasm: Interestingly, proteolysis targeting chimeras (i.e. PROTACs), developed to trigger ubiquitination and subsequent degradation of specific proteins, have recently proved to exert superior tumor growth inhibition compared to fulvestrant by targeting the estrogen receptor (ER) in a patient-derived xenograft model from a patient harboring a mutation in the ER, and a similar PROTAC is currently being tested in ER positive HER2 negative BC patients in a phase I trial (NCT04072952) [19].