 The "in vivo" oxidation and reduction of protein disulfide bonds by thiol–disulfide exchange is facilitated by a protein called thioredoxin.This small protein, essential in all known organisms, contains two cysteine amino acid residues in a vicinal arrangement (i.e., next to each other), which allows it to form an internal disulfide bond, or disulfide bonds with other proteins.As such, it can be used as a repository of reduced or oxidized disulfide bond moieties.