Antibody–drug conjugates (ADCs), such as trastuzumab deruxtecan and trastuzumab duocarmazine, have shown promising efficacy data in patients with metastatic HER2-low BC, attributed to the retention of all trastuzumab anti-tumor properties, associated with a bystander killing effect, which allows targeting and killing cancer cells even in tumors with lower degrees and heterogeneity of HER2 expression, a once limiting step for the clinical activity of anti-HER2 agents [20,21]. The gene discussed is ERBB2; the disease is breast cancer.