Glutaredoxin (Grx), also known as thiol transferase, is a small heatstable oxidoreductase. Grxs form part of the glutaredoxin system, comprising NADPH, GSH and glutathione reductase, which transfers electrons from NADPH to glutaredoxins via GSH. First recovered in E.coli as GSH-dependent hydrogen donors for ribonucleotide reductase, Grx catalyzes GSH-disulfide oxido-reductase via two redox-active cysteine residues. The active sequence (Cys-Pro-Tyr-Cys) is conserved in a variety of species. The 12 kDa dithiol protein has a role in reduction of mixed disulfides in cells exposed to oxidative stress.