ERBB2 and breast cancer: To date, HER2 inhibitors, such as trastuzumab, pertuzumab, tucatinib, lapatinib and HER2-directed antibody–drug conjugates (such as trastuzumab deruxtecan), are the standard approaches exclusively for BC with HER2 amplification even though HER2-expressing cells also exist in HER2–/HER2lo BCs13,14.