When phosphorylated, plays a role in filament reorganization. Heterotetramer of two type I and two type II keratins. Keratin-18 associates with keratin-8 (By similarity). Expressed in stratified and complex epithelia such as epidermis, mouth and pharyngeal mucosa, mucosa of anterior esophagus, rectal mucosa and gland canals, gill mucosae, gill squamous epithelium and secondary lamellae, uterus mucosa, cornea, conjunctiva, outermost layer of Sclera and in olfactory epithelium. Also expressed in simple epithelia and epitheloid cells such as lateral line canal (mantle cells, neuromast), dura mater (meninges) and ampullae of Lorenzini. Phosphorylated. Proteolytically cleaved by caspases during epithelial cell apoptosis. There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.