Microscopy of keratin filaments inside cells.

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".

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.