Cytokeratins (CK) are intermediate filaments of epithelial cells, both in keratinizing tissue (ie., skin) and non-keratinizing cells (ie., mesothelial cells). Although not a traditional marker for endothelial cells, cytokeratins have also been found in some microvascular endothelial cells. Atleast 20 different cytokeratins (CK) in the molecular range of 40-70 kDa and isoelectric points of 5-8.5 can be identified using two dimensional gel electrophoresis. Biochemically, most members of the CK family fall into one of two classes, type I (acidic polypeptides) and type II (basic polypeptides). At least one member of the acidic family and one member of the basic family is expressed in all epithelial cells. Monoclonal antibodies to cytokeratin proteins can be useful markers for tumor identification and classification.Reacting with a variety of keratins (4,5,6,8,10,13 and 18) which reacts with a variety of normal reactive and neoplasmic epithelia. Reacting with simple epithelium and both basal and superbasal layers of comfying and non comfying squamous epithelium this antibody is also useful in staining cultured epithelial cell lines. It is useful in differentiating epithelial tumors from non-epithelial tumors.