Cytokeratin 8 (CK8) is a member of the type II family of cytokeratins with a molecular weight of 53 kDa which plays a role in maintaining cellular structural integrity and also functions in signal transduction and cellular differentiation. It generally exists in monolayer and pseudostratified epithelium, and is expressed in glandular epithelium, transitional epithelium and hepatocytes. It is absent in squamous cell carcinomas. CK8 dimerizes with CK18 to form an intermediate filament in simple single-layered epithelial cells. CK8/18 expression patterns are useful for classification of tumors of unknown origin and poorly differentiated carcinomas.