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. May associate with KRT17. 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.