The most thoroughly characterized non-heme iron center in biology is Rubredoxin, the simplest member of the iron-sulfur class of metalloproteins. Rubredoxin contains a high-spin iron atom with tetrahedral coordination by four cysteinyl sulfur atoms. A structural variant of this center is found in Desulforedoxin, the smallest known Rubredoxin type protein. The 3D structure of both Rd and Dx has been determined at high resolution. These two proteins can therefore be used as case studies in which structural control by the polypeptide chain over the metal site can be discussed in detail.