ERBB2 and breast cancer: Recent studies found that among patients with HER2 non-amplified breast cancer, those with HER2 1+ or 2+ by IHC can also benefit from anti-HER2 ADCs (antibody-drug conjugates including ado-trastuzumab emtansine and fam-trastuzumab deruxtecan-nxki), while those without HER2 expression (HER2-zero breast cancer) cannot [6,7,8,9].