Notes: DL-Cystine-d4 is intended for use as an internal standard for the quantification of cystine by GC- or LC-MS. DL-Cystine is a racemic mixture of D- and L-cystine. D-Cystine is an inactive enantiomer of L-cystine (Item No. 31727), an oxidized form of cysteine (Item No. 33309).{61812}