From Citizendium 
Cystine refers to the oxidative linkage between two molecules of cysteine, one of the common amino acids, in the form of a disulfide bond. It may also refer to such a bond that occurs between two cysteine amino acids in a protein. Typically, intracellular proteins have few, if any, such disulfide bonds, while extracellular proteins tend to have several of them.
Such disulfide bonds are integral to the structural stability of many proteins.
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