Historically, HER2-positive breast cancer has been defined in a binary fashion as either HER2 positive, defined as a score of 3+ on immunohistochemistry (IHC) or 2+ on IHC and positive in situ hybridisation (ISH), or HER2 negative, defined by a score of 0–1 on IHC or 2+ on IHC and negative on ISH [68]. The gene discussed is ERBB2; the disease is breast cancer.