ERBB2 and breast cancer: ERBB2 (HER2) activation is an established oncogenic driver and key biomarker in multiple cancer types and is the biological basis of FDA approvals in breast cancer (trastuzumab, lapatinib, pertuzumab plus trastuzumab, trastuzumab emtansine, neratinib, T-DXd, tucatinib plus trastuzumab, and margetuximab), GEC (trastuzumab and T-DXd), NSCLC (T-DXd), CRC (tucatinib plus trastuzumab), and most recently pan-tumor (T-DXd; refs. 2–8).