May play a role in late hair differentiation. Heterotetramer of two type I and two type II keratins. Expressed in skin and scalp. In the hair follicle, it is present in the upper hair cuticle and the upper cortex. Also present in the in the upper portion of beard hairs (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.