a type I cytoskeletal keratin. The keratins are intermediate filament proteins responsible for the structural integrity of epithelial cells and are subdivided into cytokeratins and hair keratins. There are two types of cytoskeletal and microfibrillar keratin: type I (acidic; 40-55 kDa) [K9 to K20] and type II (neutral to basic; 56-70 kDa) [K1 to K8]. Both a basic and an acidic keratin are required for filament assembly. Generally associates with K1. Note: This description may include information from UniProtKB.