Plays a central role in hair and nail formation. Essential component of keratin intermediate filaments in the companion layer of the hair follicle (By similarity). Heterodimer of a type I and a type II keratin (By similarity). May associate with KRT17. Expressed in the companion layer and upper germinative matrix region of the hair follicle and the medulla of the hair shaft. Also expressed in epithelia of the nail bed and fungiform papillae of dorsal tongue epithelium (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). Its presence in surviving mice lacking Krt6a and Krt6b, may explain the absence of hair and nail defects in such mice. Belongs to the intermediate filament family.