Keratin
one of a family of fibrous structural proteins; protein that protects epithelial cells from damage or stress / From Wikipedia, the free encyclopedia
Keratin is one of a family of fibrous proteins. Keratin is what makes up hair in animals, as well as horns, nails, hooves, shells, beaks, and feathers. The name comes from Greek word keras meaning "horn".
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Keratins are tough and insoluble. They form the hard but non-mineral structures in reptiles, birds, amphibians and mammals. A similar biological toughness is found in chitin.