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.

Keratins are the main constituent of structures that grow from the skin:

• the α-keratins in the hair (including wool), horns, nails, claws and hooves of mammals
• the harder β-keratins in the scales and claws of reptiles, their shells (chelonia, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds. These keratins are formed mainly in beta sheets. However, beta sheets are also found in α-keratins.^[1]

Arthropods such as crustaceans often have parts of their exoskeleton made of keratin, sometimes in combination with chitin.

Keratins are also found in the gastrointestinal tracts of many animals, including roundworms (who also have an outer layer made of keratin).

Although it is now difficult to be certain, the scales, claws, some protective armour and the beaks of dinosaurs would, almost certainly, have been composed of a type of keratin.

The silk fibroins produced by insects and spiders are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins.

Silk found in insect pupae, and in spider webs and egg casings, also has twisted β-pleated sheets incorporated into fibers wound into larger supermolecular aggregates.

1. ↑ Kreplak L.; et al. (2004). "New aspects of the alpha-helix to beta-sheet transition in stretched hard alpha-keratin fibers". Biophys J. 87 (1): 640–7. Bibcode:2004BpJ....87..640K. doi:10.1529/biophysj.103.036749. PMC 1304386. PMID 15240497.

• Composition and β-sheet structure of silk Archived 2006-02-20 at the Wayback Machine
• Spider silk fiber structure
• Hair-Science.com's entry on the miroscopic elements of hair Archived 2006-01-13 at the Wayback Machine
• keratin antibody review Archived 2007-09-07 at the Wayback Machine