ERBB2 and breast cancer: Amplification/overexpression of human epidermal growth factor receptor 2 gene (ERBB2, formerly HER2), is associated with shortened disease-free (DFS) and overall survival (OS) in patients whose breast cancers contain this alteration.1,2,3 Because targeted therapies using anti-HER2 humanized monoclonal antibodies,4,5,6,7 small molecule inhibitors of HER2 kinase,8,9,10 and antibody-drug conjugates11,12 effectively treat patients with HER2-positive breast cancer, accurate assessment of HER2 status is critically important for treatment selection.