Essential for the proper assembly of types I and II keratin protein complexes and the formation of keratin intermediate filaments in the inner root sheath (irs). Heterotetramer of two type I and two type II keratins. Strongly expressed in skin and scalp, and weak expression observed in thymus. In the hair follicle, expressed in Henle layer, Huxley layer and in the irs cuticle. Expression extends from the bulb region up to the point of differentiation into the three layers. Also present in the medulla of beard hair (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.