DS supplement is newly developed for use in cell-free coupled transcription/translation systems PUREfrex 2.0 and PUREfrex 2.1 to promote proper disulfide bond formation. DS supplement will be phased out and replaced with DsbC Set. Formation of disulfide bonds is important for the stability and activity of many proteins, including cell surface receptors and secreted proteins. Disulfide bonds are formed by oxidation of pairs of cysteine sulfhydryl (SH-) groups. Therefore, an oxidizing environment is necessary for disulfide bond formation. Additionally, the correct pairing of cysteines is promoted by Disulfide bond isomerase (e.g. E. coli DsbC) which can catalyze disulfide bond exchange reactions. DS supplement comprises highly purified E. coli DsbC and oxidized Glutathione (CSSG).