 While free cysteine residues do occur in proteins, most are covalently bonded to other cysteine residues to form disulfide bonds, which play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium.Since most cellular compartments are reducing environments, disulfide bonds are generally unstable in the cytosol with some exceptions as noted below.Disulfide bonds in proteins are formed by oxidation of the sulfhydryl group of cysteine residues.