Cytokeratins are typical intermediate filaments of the cytoskeleton of epithelial cells. They are a biochemically highly diverse multigene family of polypeptides with molecular masses ranging from 40 to 68 kDa. Until now 20 distinct cytokeratin polypeptides have been described, which can be divided into an acidic (type I) and a neutral-basic (type II) subfamily. The cytokeratin expression profile within one cell characterizes the type of epithelia and also the degree of maturation or differentiation. Most malignant cells which have their origin in epithelial tissue express a certain cytokeratin profile, which can be used for classifying carcinomas and for distinguishing carcinomas from malignant tumors of nonepithelial origin. The Anti-Cytokeratin (CK3-6H5) antibody is a pan-cytokeratin–specific antibody targeting cytokeratins from simple epithelia. It crossblocks Cam5.2, an antibody specific for cytokeratin 7 and 8, and has been shown to react with native cytokeratin preparations containing cytokeratins 8, 18, and 19 as well as with recombinant cytokeratin 8.