ERBB2 and breast cancer: HER2-low breast cancer (BC), as defined by an immunohistochemistry (IHC) score for HER2 of 1 + , or 2+ with negative in situ hybridization (ISH), accounts for ~60% of all BCs classified as HER2-negative, and it includes a substantial proportion of patients historically classified as bearing Hormone Receptor-positive (HR) + /HER2-negative BC or triple-negative BC1.