The NifEN protein complex serves as a molecular scaffold where some of the steps for the assembly of the iron-molybdenum cofactor (FeMo-co) of nitrogenase take place. Purified NifEN protein contains multiple metal-sulfur clusters, where the exact types, structures, or functions are still unclear. Recently, Mo that is suitable for in vitro FeMo-co biosynthesis was found to be bound to purified NifEN. Molybdenum K-edge EXAFS performed on purified NifEN shows that the NifEN-bound Mo is part of a [MoFe3S3+X] ligand environment within the protein complex. The presence of a [MoFe3S3+X] cluster in NifEN has profound implications for FeMo-co synthesis.