1. Any of a class of filamentous proteins that are abundant in the cytoskeleton of vertebrate epithelial cells and are the main constituents of the outer layer of skin and tough epidermal structures such as hair, nails, hooves, feathers, and claws.
2. Material composed principally of keratin proteins.
[Greek keras, kerāt-, horn; see ker-1 in the Appendix of Indo-European roots + -IN.]
ke·rati·nous (kə-rătn-əs) adj.
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