Has a role in hair formation. Specific component of keratin intermediate filaments in the inner root sheath (IRS) of the hair follicle (By similarity). Heterotetramer of two type I and two type II keratins. Expressed in epidermis with a particularly strong staining in the nail matrix, nail bed and hyponychium (at protein level). There are two types of cytoskeletal and microfibrillar keratin, I (acidic) and II (neutral to basic) (40-55 and 56-70 kDa, respectively). Belongs to the intermediate filament family.