May serve an important special function either in the mature palmar and plantar skin tissue or in the morphogenetic program of the formation of these tissues. Plays a role in keratin filament assembly (By similarity). May be involved in spermatid nuclear shaping and sperm development. Heterotetramer of two type I and two type II keratins. Expressed in the perinuclear ring of spermatid manchettes within testis and in keratinocytes of the suprabasal layer of footpad epidermis (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). Belongs to the intermediate filament family.