In addition to conventional dual HER2-targeted monoclonal antibodies, HER2 tyrosine kinase inhibitors (TKIs), antibody–drug conjugates (ADCs) such as trastuzumab emtansine (T-DM1) or trastuzumab–deruxtecan (T-DXd) have significantly improved clinical outcomes of HER2+ BC [9,10,11]. The gene discussed is ERBB2; the disease is breast cancer.