Keratin is an insoluble protein that is the major constituent of the outer layer of the skin, nails, and hair. It contains greater amounts of amino acid called cystine.
Citing this page:
Generalic, Eni. "Keratin." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.