Heterotetramer of two type I and two type II keratins (By similarity). Forms a heterodimer with KRT14 (PubMed:24940650). Interacts with PLEC isoform 1C, when in a heterodimer with KRT14 (PubMed:24940650). Interacts with NOD2 (PubMed:27812135). Expressed in a discontinuous manner in the basal cell layer of adult skin epidermis, but continuously in the basal layer of fetal skin epidermis and nail. Also expressed in the outer root sheath above the hair bulb in hair follicle (at protein level). Expressed homogeneously in all cell layers of the esophagus and exocervix, but detected in the basal cell layer only of oral mucosa, skin and in the basal plus the next two layers of the suprabasal epithelium of the palate. 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.