KRT18 encodes the type I intermediate filament chain keratin 18. Keratin 18, together with its filament partner keratin 8, are perhaps the most commonly found members of the intermediate filament gene family. They are expressed in single layer epithelial tissues of the body. Mutations in this gene have been linked to cryptogenic cirrhosis. Two transcript variants encoding the same protein have been found for this gene. Cytokeratins 8 and 18 (CK8/18) make up one of the low molecular weight cytokeratins (LMW-CK). CK8/18 is expressed in simple, non-stratified epithelia, basal and superficial cells of transitional epithelium, the luminal/secretory cells of complex epithelia, mesothelium, and may be present in some types of mesenchymal cells. Multiple cytokeratins family members may be expressed in a given cell and are characteristic of the cell type and differentiation state. Nearly all carcinomas of epithelial origin and mesotheliomas express CK8/18, and CK8/18 expression patterns aid in the classification of tumors of unknown origin and poorly differentiated carcinomas. Antibodies to CK8/18 may be useful for classification of tumors of epithelial origin.