Frataxin (FXN) specifically binds 2 atoms of ferrous iron per monomer (reviewed in Stemmler et al. 2010). Iron bound to Frataxin may (Yoon and Cowan 2003, Gerber et al. 2003) or may not (Schmucker et al. 2011) enhance the interaction of Frataxin with NFS1, ICSU, and ISD11. Frataxin was shown to stimulate the cysteine desulfurase activity of NFS1 and was proposed to be a regulator of sulfur production (Tsai et al. 2010). The formation of sulfide by NFS1 is most efficiently observed when NFS1 is in complex with ISD11, ISCU, and FXN in the presence of cysteine and iron. This means that only the complete system of NFS1, ISD11, ISCU, FXN, cysteine, and iron is fully active as a desulfurase. FXN therefore seems to be a regulator of the cysteine desulfurase permitting sulfide production only when all components needed for Fe-S cluster synthesis are present and the ISCU-bound Fe-S cluster can be formed.