Currently, it is also crucial to identify a subset of HER2 non-overexpressed/non-amplified breast cancers with HER2 IHC score of 1+ or 2+ and a negative in-situ hybridization result (HER2-low) to select suitable patients for treatment with the HER2-directed antibody-drug conjugate trastuzumab deruxtecan (T-DXd) [108]. This evidence concerns the gene ERBB2 and breast cancer.