Cytokeratin 9 (CK9)

Keratin 9 is a type I cytokeratin. It is found only in the terminally differentiated epidermis of palms and soles. Mutations in the gene encoding this protein cause epidermolytic palmoplantar keratoderma.Langbein et al. (1993) reported the cloning and sequence of the KRT9 gene. The amino acid sequence showed the typical structure of type I cytokeratins, with a head (153 residues), alpha-helical coiled-coil-forming rod (306 residues), and tail (163 residues) domains. The keratin 9 protein displayed highest homology to human keratin 10 , not only in the highly conserved rod domain but also in large parts of the head and tail domains. Expression was confined to palmar and plantar epidermis. In situ hybridization and immunolocalization showed expression in subrabasal cell layers.