Due to the retention of all trastuzumab antitumor properties, associated with an improved tumor-specific cytotoxic effect, as well as the bystander killing effect, ADCs like T-Dxd and trastuzumab–duocarmazine are able to target and kill HER2-expressing BC cells even upon low-level of HER2 expression, a once limiting step for the clinical activity of anti-HER2 agents. The gene discussed is ERBB2; the disease is neoplasm.