KRT4 (Cytokeratin 4) is a member of the keratin gene family, a type II cytokeratin that is specifically expressed in differentiated layers of the mucosal and esophageal epithelia along with KRT13. Mutations in KRT4 and KRT13 are associated with White Sponge Nevus, described by oral, esophageal, and anal leukoplakia. In immunohistochemistry, KRT4 has cytoplasmic positivity in suprabasal cells of non-keratinized, stratified squamous epithelium of the esophagus and cornea, airway epithelia, and it is also expressed in the anus, larynx, pharynx, and tongue. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; J Biol Chem 1998;273:23912, PMID: 9727005; Meta Gene. Volume 2, December 2014: 374-383, DOI: 10.1016/j.mgene.2014.04.008