In this trial, patients with metastatic HER2-low breast carcinoma who had already received one or two lines of chemotherapy were randomized 2:1 to receive either trastuzumab deruxtecan (T-DXd) or the investigator’s choice of chemotherapy (capecitabine, eribulin, gemcitabine, paclitaxel). This evidence concerns the gene ERBB2 and breast carcinoma.