<b>Background:</b> HER2-low breast cancer (HER2 immunohistochemical [IHC] score 1+, or IHC 2+ without HER2 gene amplification) is distinct from HER2-positive and HER2-0 breast cancer (IHC 0), with a differing prognosis and specific therapeutic options. This evidence concerns the gene ERBB2 and breast cancer.