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 Huxley layer, but not in Henle layer or cuticle of the IRS. In the IRS Huxley layer, it is expressed in specialized Huxley cells, termed 'Fluegelzellen, along the area of differentiated Henle cells (at protein level). The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. 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.