According to the 2018 guidelines from the American Society of Clinical Oncology and the College of American Pathologists (ASCO/CAP), HER2-positive breast cancer is characterized by overexpression of HER2 in immunohistochemistry (IHC) testing score of 3+ and/or gene amplification in fluorescent in situ hybridization (FISH) testing. This evidence concerns the gene ERBB2 and breast carcinoma.