Due to the high cost of differentiating among those five subtypes using gene expression profiles, in clinical practice, immunohistochemistry (IHC) is used to classify BC according to the protein expression levels of estrogen and progesterone receptors and HER2 [the latter of which is also assessed with in situ hybridization (ISH)] into hormone receptor-positive, HER2-positive, and triple-negative BC. The gene discussed is ERBB2; the disease is breast cancer.