Essential for the proper assembly of type I and type II keratin protein complexes and formation of keratin intermediate filaments in the inner root sheath (irs). Heterotetramer of two type I and two type II keratins. Interacts with KRT6A to form filaments. Expressed in skin. Expressed in the Henle layer and cuticle of the irs in hair follicle bulb. In the hair follicle, expression was observed in all layers of the irs but was stronger in the Henle layer and cuticle than the Huxley layer until the Henle layer differentiated (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.