ERBB2 and breast cancer: Trastuzumab deruxtecan (T-DXd) is an antibody against HER2 (trastuzumab), conjugated with a topoisomerase I inhibitor (deruxtecan, a derivative of exatecan) through tetrapeptide-based linker, approved for the treatment of HER2-positive as well as HER2-low metastatic BC.