D-Cystine is the D-enantiomer of cystine, a disulfide-linked dimer of cysteine. As a non-natural amino acid, it is primarily used in biochemical and pharmaceutical research. It serves as a chiral building block in the synthesis of peptides containing D-amino acids, which are explored for their resistance to enzymatic degradation and potential in drug development. D-Cystine is also employed in studies investigating enzyme stereospecificity, protein folding, and oxidative stress mechanisms due to its structural similarity to L-cystine but differing biological activity. Unlike L-cystine, it is not suitable for nutritional supplements, food additives, or standard pharmaceutical formulations intended for human consumption.