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. At least 20 different cytokeratins (CK) in the molecular range of 40 to 70kD and isoelectric points of 5 to 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. Defects in KRT5 are a cause of epidermolysis bullosa simplex.