An alternative spelling of cystine, a sulfur-containing amino acid found in proteins that helps form strong bonds in hair, skin, and nails.
From Greek 'kystis' meaning bladder, later applied to the amino acid due to its discovery in bladder stones; the Greek root originally referred to any sac-like structure in the body.
Cystine is literally two cysteine amino acids linked together by a special sulfur bond called a disulfide bridge—this bond is so strong it's partly why hair can hold a curl after you perm it, because the heat breaks and reforms these bonds.
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