Cytokeratins are intermeiate filament keratins found in the intracytoplasmic cytoskeleton of epithelial tissue. There are two types of cytokeratins: the low weight, acidic type I cytokeratins and the high weight, basic or neutral type II cytokeratins. Cytokeratins are usually found in pairs comprising a type I cytokeratin and a type II cytokeratin. The high molecular weight cytokeratins, which are the basic or neutral cytokeratins, comprise subtypes CK1, CK2, CK3, CK4, CK5, CK6, CK7, CK8 and CK9. The low molecular weight cytokeratins, which are the acidic cytokeratins, comprise subtypes CK10, CK12, CK 13, CK14, CK16, CK17, CK18, CK19 and CK20. Expression of these cytokeratins is frequently organ or tissue specific. As an example, CK7 is typically expressed in the ductal epithelium of the genitourinary (GU) tract and CK20 most commonly in the gastrointestinal (GI) tract. Histopathologists employ such distinctions to detect the cell of origin of various tumors.