ESR1 and breast cancer: Indeed, the combined receptor blockade strategy targeting HER2 and ER is now being investigated in a non-chemotherapy (de-escalation trial design) adjuvant treatment in an ongoing phase II trial of adjuvant ET, pertuzumab, and trastuzumab (administered subcutaneously with recombinant hyaluronidase) in patients (N = 375) with anatomic stage I HR+/HER2+ breast cancer (ADEPT; NCT04569747).