Keratin n. : A nitrogenous substance, or mixture of substances, containing sulphur in a loose state of combination, and forming the chemical basis of epidermal tissues, such as horn, hair, feathers, and the like. It is an insoluble substance, and, unlike elastin, is not dissolved even by gastric or pancreatic juice. By decomposition with sulphuric acid it yields leucin and tyrosin, as does albumin. Called also epidermose.

Example:
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Taxonomy
keratins, cells, Silk, disulfide bridges, intermediate filaments, skin, alanine, mammals, glycine, human hair, Alpha helix, free encyclopedia, beta sheets, keratin protein, protein chains, chains, disease, adhesive.

Defination from the web:
keratin, sometimes in combination with chitin. ... In Crossopterygian fish, the outer layer of cosmoid scales was keratin. ...