Glutaredoxins are a family of small molecular weight proteins that have a central role in cellular redox regulation. They are kept in a reduced, active form by glutathione and glutathione reductase using NADPH. Glutaredoxins perform two distinct enzymatic reactions. First, reduction of protein disulfides depends on the active site motif Cys-X-X-Cys, using the so-called 'dithiol mechanism' similar to thioredoxins. Second, reversible (de-)glutathionylation of proteins that depends only on the N-terminal active site cysteinyl residue, using the 'monothiol mechanism'. Glutathionylation is a post-translational modification of proteins, similar to phosphorylation - thus the addition/removal of glutathione from target proteins acts as a molecular switch to regulate protein activities. (Adapted from FBrf0200723 and PMID:23397885.)