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 throughout embryonic development with highest levels at 8.5 dpc. Expression decreases by 11.5 dpc and increases again by 17.5 dpc. 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.