keratin, any one of a class of fibrous protein molecules that serve as structural units for various living tissues. The keratins are the major protein components of hair, wool, nails, horn, hoofs, and the quills of feathers. These proteins generally contain large quantities of the sulfur-containing amino acids, particulary cysteine. The helical keratin molecules twist around each other to form elongated strands called intermediate filaments. The formation of a covalent chemical bond called a disulfide bridge between the sulfer atoms on two cysteins on separate polypeptide chains of keratin allows for the cross-linkage of these chains and results in a fairly rigid aggregate. This phenomenon is seen to be consistent with the physiological role of the keratins, which provide a tough, fibrous matrix for the tissues in which they are found. Human hair is approximately 14% cystine (cysteins cross-linked by disulfide bridges).

Fibrous structural protein of hair, nails, hooves, wool, feathers, and skin. A quarter of the amino acids in keratin are cystine, whose ability to form strong bridging (disulfide) bonds with other cystine units accounts for keratin's great stability. Keratin does not dissolve in cold or hot water and does not easily undergo proteolysis. Its fibres are 10–12percnt longer at maximum water content (about 16percnt) than when dry. The sulfurous smell of burning keratin is distinctive.

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