Plays a central role in hair and nail formation. Essential component of keratin intermediate filaments in the companion layer of the hair follicle. Heterodimer of a type I and a type II keratin. May associate with KRT17. Highly expressed in hair follicles from scalp. Specifically expressed in the of the hair companion layer follicle, a single layered band of flat and vertically oriented cells between the cuboidal outer root sheath (ORS) cells and the inner root sheath (IRS) that stretches from the lowermost bulb region to the isthmus of the follicle. Also expressed in medullated hairs. In nails, it is almost exclusively present in the nail bed (at protein level). The Thr-161 variant may increase risk to develop pseudofolliculitis barbae (PFB) [MIM:612318]. PFB is a common hair disorder characterized by a pustular foreign body inflammatory reaction that is induced by ingrown hairs of the facial and submental (barbea) regions after regular shaving. It occurs predominantly in black males, while it is rather rare and usually far less severe in Caucasian males. The disease may be caused by variants affecting the gene represented in this entry. May be used as a marker of hair differentiation. 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.