Heterotetramer of two type I and two type II keratins. Strongly expressed in skin and scalp, and weak expression observed in thymus and tongue. In the hair follicle, expression is restricted to the mid- to upper inner root sheath cuticle, being present slightly above the apex of the dermal papilla (at protein level). 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.