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. Specifically expressed in the inner root sheath (IRS) of the hair follicle. Present in Henle and the Huxley layers of the IRS, while expression in the cuticle is unsure (at protein level). Expressed exclusively in the inner root sheath (IRS) of anagen hair follicles, where expression is predominantly in the hair cone during anagen III and in the Huxley and Henle layers of the inner root sheath during anagen VI. Mice exhibit defects in hair structure and progressive alopecia. Missense mutations cause milder phenotypes such as caracul, characterized by rough and greasy fur, and wavy hair that is pointed in different directions. 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.