Keratins are cytoplasmic intermediate filament proteins expressed by epithelial cells. CK13 is a type I keratin and functions as a marker for esophageal type differentiation which is expressed by various internal stratified epithelia. Expression is also found in some epidermal sweat gland ducts (at protein level) and in exocervix, esophagus and placenta. CK13 has been used as a marker for non-keratinized squamous epithelium and is also expressed in various squamous metaplasia, but it is down regulated in squamous dysplasia and squamous carcinoma.