Keratin

• 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.

Keratin molecules are helical and fibrous, twisting around each other to form strands called intermediate filaments. These proteins contain a high percentage of sulfur-containing amino acids, largely cysteine, which form disulfide bridges between the individual molecules resulting in a fairly rigid structure. Human hair is approximately 14% cysteine.