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. In the hair follicle and bulb, uniformly expressed in all three layers of the inner root sheath (the Henle layer, the Huxley layer and the cuticle) and observed in matrix cells (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.