ERBB2 and breast cancer: Furthermore, our study does not address “HER2-low” breast cancer, a potentially large subgroup of BC that is poorly defined37 as having low to moderate levels of HER2 expression (always ISH-negative but either IHC 1+ or IHC 1+ / 2+) and that has been shown to generally respond to antibody-drug conjugates such as trastuzumab-deruxtecan (T-DXd) in the metastatic BC setting38.