FeS clusters are essential cofactors for life facilitating electron transport reactions. FeS cluster biosynthesis is mainly located in mitochondria and utilizes several enzymes. Often cysteine residues are ligands for FeS clusters. We identified glutaredoxins as FeS proteins that uses the cysteine residue of a bound glutathione as non-protein ligand. Our research wants to identify further FeS oxidoreductases in all kinds of organisms and to elucidate the function of these unusal FeS proteins.