We also investigated the efficacy of the combination of lumretuzumab (an anti-HER3 antibody [37–39]) and pertuzumab (an anti-HER2 antibody [40]) with a selective estrogen receptor degrader (fulvestrant) in patient-derived mouse xenograft models of HER3+/HER2-low/ER+ breast cancer. The gene discussed is ESR1; the disease is breast carcinoma.