▐ker-ә-tәn\ n [ISV] (ca. 1849) (1) The protein derived from feathers, hair, hoofs, horns, etc of animals by calcinations. It is sometimes used as filler in plastics, particularly urea-formaldehyde molding compounds, in which it reduces brittleness and permits drilling and tapping. (2) A class of natural fibrous proteins occurring in vertebrate animals and man, characterized by their high content of several amino acids, especially crystine, arginine, and serine. They are generally harder than the fibrous collagen group of proteins. Keratins are insoluble in organic solvents but do absorb and hold water. The molecules contain both acidic and basic groups and are thus amphoteric. (Morrison RT, Boyd RN (1992) Organic chemistry, 6th edn. Prentice Hall, Englewood Cliffs, NJ).