The phase 3 DESTINY-Breast04 study demonstrated that trastuzumab deruxtecan could significantly prolong overall survival compared with the physician’s choice of chemotherapy in patients with HER2 IHC 2+ with ISH-negative or IHC 1+ metastatic breast cancer, which was regarded as HER2-negative by the previous breast cancer definition [64]. The gene discussed is ERBB2; the disease is breast carcinoma.