Heterotetramer of two type I and two type II keratins. Interacts with NOD2 (By similarity). Expressed in the basal cell layers of several stratified epithelia including esophagus, tongue, stomach, epidermis and hair follicle. In the hair follicle, expression is detected mainly in the basal layer of the outer root sheath (ORS), except just above the follicle bulb where it occurs throughout its thickness. Low expression levels are seen in the single layer of ORS cells around the base of the follicle which increases in the palisade-like cells of the bulb. Also expressed in the basal cells of the sebaceous glands, and expression in the epidermis occurs in a punctate pattern. 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.