Cytokeratin polypeptide 19 (CK19) is a type I intermediate filament protein that is expressed in stratified and simple-type epithelia. CK19 is synthesized mainly in embryonic and adult simple epithelia, but has also been found in non-keratinizing stratified epithelia as well. CK19 is the smallest known keratin and is remarkable in that, contrary to all other keratins, it does not have a designated partner for the formation of filaments, implying that regulation of its expression is different from other keratin-encoding genes. CK19 antibody is a useful tool for the identification of epithelial tumors. It is helpful in distinguishing hepatocellular carcinoma (CK19-) from cholangiocarcinoma (CK19+) or metastatic carcinoma in liver (CK19+).