Keratins are a family of intermediate filament proteins that assemble into filaments through forming heterodimers of one type I keratin (keratins 9 to 23) and one type II keratin (keratins 1-8). The two keratin types share only 30% sequence homology. Keratins demonstrate tissue- and differentiation-specific expression profiles. Cytokeratin 18 is a type I intermediate filament protein that exists as a heterotetramer composed of two type I and two type II keratins. Cytokeratin 18 is expressed in all simple type epithelia and basal cells of many squamous and non-epidermal epithelia and is often co-expressed with cytokeratin 8. Tissues from the gastrointestinal tract, respiratory tract, and urogenital tract as well as endocrine and exocrine tissues and mesothelial cells are positive for cytokeratin 18.