ESR1 and breast cancer: The same ER+/HER2-low/HER3+ human breast cancer HBCx-19 xenograft model used previously was treated with lumretuzumab (3 mg/kg i.p.), pertuzumab (3 mg/kg i.p.), and the anti-estrogen fulvestrant (50 mg/kg i.m.), either as single-agents or in combination.