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Keratin 4 is a type II cytokeratin. It is specifically found in differentiated layers of the mucosal and esophageal epithelia together with keratin 13. Mutations in the genes encoding this protein have been associated with White Sponge Nevus, characterized by oral, esophageal, and anal leukoplakia.Keratin 4 partners with a similar protein, keratin 13 （produced from the KRT13 gene）, to form molecules known as intermediate filaments. These filaments assemble into strong networks that provide strength and resilience to the different mucosae. Networks of intermediate filaments protect the mucosae from being damaged by friction or other everyday physical stresses.