According to the 2018 ASCO and College of American Pathologists (CAP), HER2 protein overexpression assessed by immunohistochemistry (IHC) score 3+ or HER2 gene amplification, assessed by in situ hybridization (ISH) assay, is the predictive biomarker of HER2-targeted therapies in breast cancer, such as trastuzumab. The gene discussed is ERBB2; the disease is breast carcinoma.