Multiple HER2-targeted therapies have been approved to treat HER2 amplification and/or overexpression in breast cancer via different mechanisms of action, including monoclonal antibodies (mAb; e.g., trastuzumab), tyrosine kinase inhibitors (TKI, e.g., neratinib), and antibody–drug conjugates [ADC, e.g., trastuzumab emtansine and trastuzumab deruxtecan (T-DXd); ref. 2]. This evidence concerns the gene ERBB2 and breast cancer.