Around 15-20% of all breast cancers (BCs) overexpress HER2.18 According to the American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines, BCs are defined as “HER2-positive” if HER2 expression by immunohistochemistry (IHC) is scored 3+ or 2+ with gene amplification detected by in situ hybridization (ISH). The gene discussed is ERBB2; the disease is breast cancer.