<h4>Background</h4>Breast cancer was previously categorized as human epidermal growth factor receptor 2 (HER2)-positive (immunohistochemistry [IHC] 3+, IHC 2+ / in situ hybridization [ISH]-positive) or HER2-negative (IHC 0, IHC 1+, IHC 2+/ISH-). This evidence concerns the gene ERBB2 and breast cancer.