Implementation of the 2013 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guideline on HER2 testing in breast cancer, compared with the previous guideline from 2007, resulted in increases in the proportion of HER2-positive breast cancers and the proportion of cases with HER2-equivocal results [11–14]. This evidence concerns the gene ERBB2 and breast cancer.