KRT5 (Cytokeratin 5, CK5) is a protein that dimerizes with keratin 14 to form the intermediate filament that form the cytoskeleton of basal epithelial cells. It is useful for identifying myoepithelial cells and basal cells in the breast, skin, salivary glands, and prostate. It is present in the skin, the mucosa of the oral cavity, oropharynx, hypopharynx, and larynx, and in mitotically active basal cells of the skin. It is commonly positive in squamous carcinomas of the head and neck, as well as in mesotheliomas. Mutations in KRT5 cause epidermolysis bullosa simplex. Commonly used with Cytokeratin 6, the CK5/6 marker is useful for the identification of mesotheliomas as well as identifying the presence of basal epithelium in benign glands versus malignant prostate and breast carcinomas, where the basal layer is lost. References: Int J Clin Exp pathol 2008 1(1):19; Histopathology 1997 31:140; J Cell Sci 1994 107(4):765