ERBB2 and breast cancer: In vitro studies demonstrate that the antibody portion of trastuzumab deruxtecan, sharing the same amino acid sequence as trastuzumab, also binds to FcγRIIIa and complement C1q, mediating antibody-dependent cellular cytotoxicity (ADCC) in human breast cancer cells overexpressing HER2 (Hudis, 2007; Greenblatt and Khaddour, 2022).