Keratin 40 (KRT40) is a type I (acidic) keratin protein encoded by the KRT40 gene located on human chromosome 17q21.2. As a member of the intermediate filament (IF) protein superfamily, keratin 40 contributes to the cytoskeletal framework of epithelial cells, forming heteropolymeric filaments that provide mechanical support and structural integrity to tissues such as hair follicles, wool fibers, and certain glandular epithelia. KRT40 is expressed at relatively low levels under normal physiological conditions, but its expression becomes significant in specialized contexts. In hair biology, KRT40 is considered a hub gene involved in hair follicle development, cell cycle regulation, and melanogenesis, with its transcripts localized specifically to the fiber cuticle in species like sheep and yak, indicating a structural role in hair shaft formation.