Several clinical trials, including the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-31, the North Central Canter Treatment Group (NCCTG) N9831 and DESTINY-Breast04, demonstrated that a subset of breast cancer patients with HER2 low levels of expression also benefited from anti-HER2 targeted therapy, especially trastuzumab-deruxtecan (T-DXd) [13]. The gene discussed is ERBB2; the disease is breast cancer.