Cysteineglutathione disulfide is a molecule that is formed upon oxidative stress of glutathione, that will form mixed disulfides with protein thiol groups, causing reversible S-glutathionylation. S-glutathionylation is an important post-translational modification responsible for transducing oxidant signals. S-glutathionylation of thiols confers protection against their irreversible oxidation, like for instance the formation of sulphonic acid moieties. If the targeted cysteine is a functionally critical amino acid, S-glutathionylation will however also modify protein function. S-sulfonation and S-thiolation of transthyretin Phe33Cys has been detected in a patient with familial transthyretin amyloidosis. In Cystinotic human skin fibroblasts in tissue culture there is an accumulation of cystine. Stored cystine in cystinotic tissues may derive in part from glutathione-cysteine mixed disulfide via transpeptidation. Cystinosis is an autosomal recessive disorder caused by an impaired transport of cystine out of lysosomes.