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. 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, it is expressed between the lowermost bulb region of the cuticle and the region where Henle cells undergo abrupt terminal differentiation. Detected up to the uppermost cortex region where cuticle cells terminally differentiate (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.