In this study, we compared quantitative HER2 expression by the HERmarkTM Breast Cancer Assay (HERmark) with routine HER2 testing by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), and correlated HER2 results with overall survival (OS) of breast cancer patients in a multicenter Collaborative Biomarker Study (CBS). The gene discussed is ERBB2; the disease is breast cancer.