Plays a central role in hair formation. Essential component of keratin intermediate filaments in the inner root sheath (IRS) of the hair follicle. Heterodimer of a type I and a type II keratin. Associates with KRT16 and/or KRT17 (By similarity). Highly expressed in hair follicles from scalp. Specifically expressed in the inner root sheath (IRS) of the hair follicle. Present in the all 3 IRS layers: the cuticle, the Henle and the Huxley layers. Also detected in the pseudopods of specialized Huxley cells, termed Fluegelzellen, along the area of differentiated Henle cells (at protein level). In all 3 IRS layers, expression begins simultaneously in adjacent cells of the lowermost bulb above the germinative cell pool and terminated higher up in the follicle with the asynchronous terminal differentiation of each cell layer (at protein level). 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.