Heterotetramer of two type I and two type II keratins. Keratin-3 associates with keratin-12. Cornea specific. Expressed in the basal cells of corneal epithelium and stroma. Also expressed in esophageal epithelium. There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.