Keratin-14: A type I keratin that is found associated with the KERATIN-5 in the internal stratified EPITHELIUM. Mutations in the gene for keratin-14 are associated with EPIDERMOLYSIS BULLOSA SIMPLEX.Keratin-1: A type II keratin that is found associated with the KERATIN-10 in terminally differentiated epidermal cells such as those that form the stratum corneum. Mutations in the genes that encode keratin-1 have been associated with HYPERKERATOSIS, EPIDERMOLYTIC.Keratin-10: A type I keratin that is found associated with the KERATIN-1 in terminally differentiated epidermal cells such as those that form the stratum corneum. Mutations in the genes that encode keratin-10 have been associated with HYPERKERATOSIS, EPIDERMOLYTIC.Keratin-18: A type I keratin found associated with KERATIN-8 in simple, or predominately single layered, internal epithelia.Keratin-2: A type II keratin found expressed in the upper spinous layer of epidermal KERATINOCYTES. Mutations in genes that encode keratin-2A have been associated with ICHTHYOSIS BULLOSA OF SIEMENS.Keratin-4: A type II keratin that is found associated with the KERATIN-13 in the internal stratified EPITHELIUM. Defects in gene for keratin-4 are a cause of HEREDITARY MUCOSAL LEUKOKERATOSIS.Keratin-9: A type II keratin found predominantly expressed in the terminally differentiated EPIDERMIS of palms and soles. Mutations in the gene for keratin 9 are associated with KERATODERMA, PALMOPLANTAR, EPIDERMOLYTIC.Keratin-13: A type I keratin that is found associated with the KERATIN-4 in the internal stratified EPITHELIUM. Defects in gene for keratin 13 cause HEREDITARY MUCOSAL LEUKOKERATOSIS.Keratin-5: A type II keratin that is found associated with the KERATIN-14 in the internal stratified EPITHELIUM. Mutations in the gene for keratin-5 are associated with EPIDERMOLYSIS BULLOSA SIMPLEX.Keratin-3: A type II keratin that is found associated with the KERATIN-12 in the CORNEA and is regarded as a marker for corneal-type epithelial differentiation. Mutations in the gene for keratin-3 have been associated with MEESMANN CORNEAL EPITHELIAL DYSTROPHY.Keratin-16: A type I keratin expressed in a variety of EPITHELIUM, including the ESOPHAGUS, the TONGUE, the HAIR FOLLICLE and NAILS. Keratin-16 is normally found associated with KERATIN-6. Mutations in the gene for keratin-6 have been associated with PACHYONYCHIA CONGENITA, TYPE 1.Keratin-17: A type I keratin found associated with KERATIN-6 in rapidly proliferating squamous epithelial tissue. Mutations in the gene for keratin-17 have been associated with PACHYONYCHIA CONGENITA, TYPE 2.Keratins: A class of fibrous proteins or scleroproteins that represents the principal constituent of EPIDERMIS; HAIR; NAILS; horny tissues, and the organic matrix of tooth ENAMEL. Two major conformational groups have been characterized, alpha-keratin, whose peptide backbone forms a coiled-coil alpha helical structure consisting of TYPE I KERATIN and a TYPE II KERATIN, and beta-keratin, whose backbone forms a zigzag or pleated sheet structure. alpha-Keratins have been classified into at least 20 subtypes. In addition multiple isoforms of subtypes have been found which may be due to GENE DUPLICATION.Keratin-15: A type I keratin found in the basal layer of the adult epidermis and in other stratified epithelia.Keratin-12: A type I keratin that is found associated with the KERATIN-3 in the CORNEA and is regarded as a marker for corneal-type epithelial differentiation. Mutations in the gene for keratin-12 have been associated with MEESMANN CORNEAL EPITHELIAL DYSTROPHY.Keratin-8: A type II keratin found associated with KERATIN-18 in simple, or predominately single layered, internal epithelia.Keratin-6: A type II keratin found associated with KERATIN-16 or KERATIN-17 in rapidly proliferating squamous epithelial tissue. Mutations in gene for keratin-6A and keratin-6B have been associated with PACHYONYCHIA CONGENITA, TYPE 1 and PACHYONYCHIA CONGENITA, TYPE 2 respectively.Keratin-7: A type II keratin found associated with KERATIN-19 in ductal epithelia and gastrointestinal epithelia.Keratinocytes: Epidermal cells which synthesize keratin and undergo characteristic changes as they move upward from the basal layers of the epidermis to the cornified (horny) layer of the skin. Successive stages of differentiation of the keratinocytes forming the epidermal layers are basal cell, spinous or prickle cell, and the granular cell.Keratin-20: A type I keratin expressed predominately in gastrointestinal epithelia, MERKEL CELLS, and the TASTE BUDS of the oral mucosa.Keratin-19: A type I keratin found associated with KERATIN-7 in ductal epithelia and gastrointestinal epithelia.Mice, Transgenic: Laboratory mice that have been produced from a genetically manipulated EGG or EMBRYO, MAMMALIAN.Epidermis: The external, nonvascular layer of the skin. It is made up, from within outward, of five layers of EPITHELIUM: (1) basal layer (stratum basale epidermidis); (2) spinous layer (stratum spinosum epidermidis); (3) granular layer (stratum granulosum epidermidis); (4) clear layer (stratum lucidum epidermidis); and (5) horny layer (stratum corneum epidermidis).Skin: The outer covering of the body that protects it from the environment. It is composed of the DERMIS and the EPIDERMIS.Epithelial Cells: Cells that line the inner and outer surfaces of the body by forming cellular layers (EPITHELIUM) or masses. Epithelial cells lining the SKIN; the MOUTH; the NOSE; and the ANAL CANAL derive from ectoderm; those lining the RESPIRATORY SYSTEM and the DIGESTIVE SYSTEM derive from endoderm; others (CARDIOVASCULAR SYSTEM and LYMPHATIC SYSTEM) derive from mesoderm. Epithelial cells can be classified mainly by cell shape and function into squamous, glandular and transitional epithelial cells.Cell Differentiation: Progressive restriction of the developmental potential and increasing specialization of function that leads to the formation of specialized cells, tissues, and organs.