Cytokeratins are a subfamily of intermediate filament proteins and are characterized by a remarkable biochemical diversity, represented in epithelial tissues by at least 20 different polypeptides. They range in molecular weight between 40kD and 68kD and isoelectric pH between 4.9-7.8. The individual cytokeratin polypeptides are designated 1 to 20. Cytokeratin 1 has the highest molecular weight and the highest isoelectric pH, while cytokeratin 19 has the lowest molecular weight and a low isoelectric pH. The cytokeratins are divided into the type I and type II subgroups, the type II family members comprising the basic to neutral cytokeratins 1-8, while the type I group comprises the acidic cytokeratins 9-20. The cytokeratins are expressed in combinations which characterize the type of epithelium. Cytokeratin 19 is the smallest human cytokeratin (40kD) and is found in many simple and invariably in some non-keratinizing stratified squamous epithelia. All adenocarcinomas tested have been positive for cytokeratin 19.