May play a role in late hair differentiation. Heterotetramer of two type I and two type II keratins. Expressed in skin and scalp. Also very weakly expressed in tongue, breast, colon and small intestine. In the hair follicle, it is specifically present in the upper hair cuticle. Not present in the upper cortex (at protein level). During differentiation of the hair, it is one of the last keratins expressed. 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.