ERBB2 and breast cancer: Trastuzumab deruxtecan (T-DXd; formerly DS-8201a), composed of trastuzumab and a topoisomerase I inhibitor, could yield about an ORR of 37% with a median duration of response (DoR) of 10.4 months in 54 HER2-low breast cancer patients with disease refractory to standard therapies [16].