Keratins are cytoplasmic intermediate filament proteins expressed by epithelial cells. Cytokeratin 8 is a type II keratin and Cytokeratin 18 is a type I keratin. It is the major keratin pair in most simple epithelium including thyroid, female breast, gastrointestinal tract and respiratory tract. CK8 detects most non-squamous epithelial tumors and adenocarcinomas of the breast, ovary, gastrointestinal tract, thyroid, pancreas, bile duct and salivary glands. CK18 recognizes squamous carcinomas. These cytokeratins are absent from keratinizing squamous carcinomas.