According to the 2018 American Society of Clinical Oncology–College of American Pathologists (ASCO–CAP) clinical practice guidelines on HER2 testing in breast cancer, tumors are classified as HER2-positive (defined as IHC 3+ or IHC 2+/ISH+) or HER2-negative (defined as IHC 0, IHC 1+, or IHC 2+/ISH−; Figure 1) [3]. This evidence concerns the gene ERBB2 and breast cancer.