ESR1 and breast cancer: Our preclinical data also show that a triplet combination consisting of anti-HER3 (lumretuzumab), anti-HER2 (pertuzumab), and endocrine (fulvestrant) therapy achieved long-lasting tumor responses in vivo, whereas our case study demonstrates a clinical benefit of combined lumretuzumab and pertuzumab in a ER+/HER2-low breast cancer patient.