Examples include the active sites of a number of enzymes:
* Nitrogenase include two P-clusters ([8Fe-7S]) and two FeMocos ([7Fe-9S-C-Mo-R homocitrate]).
* Carbon monoxide dehydrogenase and acetyl coenzyme-A synthase each features an Fe-N-iS clusters.
* [FeFe]-hydrogenase features an "H-cluster", consisting of a FeS bridge to Fe via a cystine. The Fe half features unique ligands: 3 CO, 2 CN, and an azadithiolate  HN(CHS).
* A special 6 cysteine-coordinated [FeS] cluster was found in oxygen-tolerant membrane-bound [NiFe] hydrogenases.
* The "double cubane cluster" [FeS], found in some nitrogenase-related ATPases, consists of two [FeS] bridged by a cysteine. The functions of such proteins remain unclear.