NifB catalyzes one of the most fascinating reactions in bioinorganic chemistry. It starts by methylating an iron-sulfur cluster, followed by radical-based abstraction of all methyl hydrogen atoms to generate a carbide ion, the fusion of two iron-sulfur clusters, and the final insertion of a sulfide atom. The product is a uniquely complex Fe8S9C cluster, called NifB-co, that constitutes the inorganic core of the active site cofactors of all types of nitrogenases. NifB is thus required for all biological nitrogen fixation activity on Earth. This work presents the first 3D atomic structure of NifB and shows how this sequence of events is regulated by a short loop of amino acid residues that connects the protein active site to one of its iron-sulfur cluster substrates.