Keratin is a label for a group of fibrous proteins. They are the key component of hair, nails and the outer layer of skin. They also reinforce the parts of the mouth responsible for breaking down food in the mouth - the tongue and hard palate.

Unfortunately, some fungi, particularly those responsible for athlete's foot and ringworm feed on keratin. In addition, several genetic conditions involve a mutation in one of the genes that form keratin.

Keratin at Wikipedia

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