Cytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. Cytokeratin represents an excellent marker for the discrimination of specific subtypes of adenocarcinoma. Keratin 9 is a type I cytokeratin. It is expressed in the terminally differentiated epidermis of palms and soles. Defects in KRT9 are a cause of epidermolytic palmoplantar keratoderma (EPPK). EPPK is an autosomal dominant disease characterized by diffuse thickening of the epidermis on the entire surface of palms and soles sharply bordered with erythematous margins.