In the 1970s, ferredoxin was demonstrated to contain FeS clusters and later nitrogenase was shown to contain a distinctive MoFeS active site.  The Fe-S clusters mainly serve as redox cofactors, but some have a catalytic function. In the area of bioinorganic chemistry, a variety of Fe-S clusters have also been identified that have CO as ligands.
FeMoco, the active site of most nitrogenases, features a FeMoSC cluster.