Trastuzumab deruxtecan is an antibody–drug conjugate composed of the anti-HER-2 monoclonal antibody trastuzumab and a cytotoxic payload, SN-38 or deruxtecan, which inhibits topoisomerase I. It has demonstrated major survival benefits, initially in HER-2-positive breast cancer [65]. This evidence concerns the gene ERBB2 and breast cancer.