Cytokeratins are a family of water-insoluble intermediate filaments, composed of polypeptide chains found in the cytoskeleton of epithelial cells. Twenty different forms of human cytokeratins exist (Moll numbers 1–20) in both normal and malignant epithelial cell lines. (1) Specific combinations of cytokeratin peptides are associated with certain epithelial cells, and are therefore useful in the characterization of poorly differentiated carcinoma. (2–4) Cytokeratins may also be clinically significant indicators in distinguishing carcinoma from malignant tumors of non-epithelial origin, such as lymphoma, sarcoma, and melanoma. Cytokeratin 8 and 18 staining has also been observed in renal cell carcinomas, hepatocellular carcinomas, cholangiocarcinomas, germ cell tumors and some neuroendocrine carcinomas, epithelioid sarcomas and non-epithelial tumors. (7)