Heterodimer of a type I and a type II keratin. Colocalizes with KRT8/KRT18 filament network. Expressed in nail matrix and nail bed epithelium (at protein level). Also expressed in tongue and digits with weak expression in vibrissae and in both filiform and fungiform papillae of oral mucosa. 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.