Cytokeratin 13 (CK13), also known as keratin 13 (KRT13), is a type I acidic polymer metamorphine. It forms a heterotetramer with type I CK4 and forms an intermediate wire, mainly to shape the cell skeleton of specific epithelial cells. It is mainly expressed in suprabasal layers of noncornified stratified epithelium, including squamous (nonkeratinized) epithelia of cervix, esophagus, larynx, oral cavity, tonsils, urothelium and respiratory type epithelium. It is a marker of mature but nonkeratinized squamous epithelium.