Keratins are a large family of proteins that form the intermediate filament cytoskeleton of epithelial cells. Keratin expression is highly regulated,tissue specific,and varies according to cell-state. Type I keratins consist of acidic,low molecular weight proteins with MW ranging from 40 kDa (KRT19) to 64 kDa (KRT9). Type 2 keratins consist of basic or neutral,high molecular weight proteins with MW from 52 kDa (KRT8) to 67 kDa (KRT18). Keratin 15 is a type I cytokeratin. It is found in some progenitor basal cells within complex epithelia.