Keratin is an insoluble, sulfur-containing fibrous protein that serves as the main structural molecule in hair and nails and is also present in the skin. It is synthesized by cells known as keratinocytes and may be hard or soft. Hard keratin is found in hair and nails; soft keratin is found in the epidermis of the skin in the form of flattened non-nucleated scales that slough continually. Keratinization is the process of transforming epidermal cell material into keratin.

The main differences in various keratins arises from their sulfur content. If there are many cysteine disulfide (S-S) cross-links (see cross-linking), then there is very little flexibility as in horns, claws, hooves, or nails. In wool, skin, and muscle proteins, there are fewer disulfide cross-links, which allows some stretching but returns to normal upon relaxation of tension.