Cystine: An amino acid that is particularly notable because it is the least soluble of all of the naturally occurring amino acids and because it precipitates out of solution in the genetic disease cystinuria to form stones in the urinary tract. Cystine is the chief sulfur-containing compound in protein. Cystine is generated by the union of two cysteine molecules and so is sometimes called dicysteine. It is abbreviated Cys-Cys. See also: Cystinuria.