Keratins are a family of highly homologous proteins expressed as pairs of acidic and basic forms which make intermediate filaments in epithelial cells. Cytokeratin 13 (CK13) is the major acidic keratin, which together with CK4, its basic partner, is expressed in the suprabasal layers of non-cornified stratified epithelia including tongue mucosa, esophagus, anal canal epithelium, tracheal epithelium, uterine cervix, and urothelium. CK13 has been used as a marker for non-keratinized squamous epithelium. It is also expressed in various squamous metaplasia, but it is down regulated in squamous dysplasia and squamous carcinoma.