What protein makes up hair and nails?

The protein that makes up hair and nails is keratin. Keratin is a family of tough, fibrous structural proteins found in the hair, nails, outer skin, claws, hooves, feathers, scales, and horns of vertebrates.

Human hair and nails are made largely from hard, keratinized cells. These cells are packed with keratin filaments, which give the tissues strength and resistance to water. Keratin contains substantial amounts of the sulfur-containing amino acid cysteine. Chemical links between cysteine residues, called disulfide bonds, help stabilize the protein and contribute to the toughness of hair and nails.

Hair is more flexible than fingernails because its keratin generally has fewer interchain disulfide bonds. This difference helps explain why nails are harder while hair can bend and stretch to a greater degree. Hair and nails are not living tissue at their visible ends; growth occurs from living cells in the hair follicle and nail matrix, which produce new keratinized material.

A common mix-up is collagen, another important structural protein. Collagen is abundant in connective tissues such as skin, tendons, cartilage, and bone, but keratin is the defining structural protein of hair and nails. Melanin, meanwhile, contributes color rather than strength.