Cytokeratin 17 (CK17) is an intermediate filament protein expressed in the basal cells and myoepithelial cells of complex epithelia, including glandular epithelium with myoepithelial component, transitional and pseudostratified epithelia. CK17 is a marker in the identification of breast cancer with basal phenotype. Squamous cell carcinoma is also labeled by CK17 antibody. Antibody against CK17 may be an aid in distinguishing cholangiocarcinoma (CK17+) from hepatocellular carcinoma (CK17-). In combination with MUC1, CK17 antibody has been found to be useful in distinguishing pancreatobiliary adenocarcinoma (CK17+) from extrapancreatobiliary nonmucinous adenocarcinoma (CK17-).