KeratinKeratin is a protein used by numerous groups of animals as a structural element, and is a classic example of a fibrous protein.
- mammal hair,
- reptile scales, (though not fish scales),
- bird feathers,
- claws (including nails and hooves),
- horn (but not antlers),
- baleens (the sieve-like structures with which whales filter food out of water),
- the enamel of the teeth.