ERBB2 and breast cancer: Interestingly, recent studies have demonstrated that about 60% of metastatic breast cancers categorized as HER2-negative, whether HR-positive or HR-negative, express low levels of HER2 that is — 1) measurable with an immunohistochemistry (IHC) score of 1+ or 2+ and negative in situ hybridization (ISH) results and 2) targetable with ADCs like T-DXd.