Involved in the organization of myofibers. Together with KRT8, helps to link the contractile apparatus to dystrophin at the costameres of striated muscle (By similarity). Heterotetramer of two type I and two type II keratins. Interacts with PNN and the actin-binding domain of DMD (By similarity). Expressed in the digestive tract throughout development in the epithelia of proventriculus and glandular structures. In 13 day embryo, strongly expressed in esophagus with moderate expression in proventriculus and lung. Weak expression in gizzard, small intestine, lung and dorsal skin. In embryos over 13 days old, observed in ectodermal, endodermal epithelium and also in neural and mesodermal tissues (i.e. notochord), floorplate in the neural tube, somatic mesoderm, splanchnic mesoderm and dermatome. In more developed embryos, expression was localized in the anterior lobe of the pituitary gland, notochord and hypothalamus of the diencephalon (derived from the floor plate of the neural tube). Also localized in the mesonephric mesoderm and lateral plate mesoderm. This keratin differs from all other IF proteins in lacking the C-terminal tail domain. 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.