Has a role in hair formation. Specific component of keratin intermediate filaments in the inner root sheath (IRS) of the hair follicle (Probable). Heterotetramer of two type I and two type II keratins. Highly expressed in hair follicles from scalp and eyebrow. Also expressed in palmoplantar epidermis. Not expressed in face skin despite the presence of fine hairs histologically. In hair, it is specifically present in the inner root sheath (IRS) of the hair follicle. Present in the IRS cuticle, but not in Henle or Huxley layers of the IRS. In the IRS cuticle, its presence is delayed up to the height of the apex of the dermal papilla (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. Was initially thought to be the ortholog of mouse KRT71.