<p>Members of this protein family, to date, are found in a completed prokaryotic genome if and only if the species is one of the archaeal methanogens. The presence of motifs with seven invariant Cys residues in the N-terminal 50 residues, including three instances of CXXC, would be consistent with function as an oxidoreductase with FeS clusters. The exact function is unknown, but likely is linked to methanogenesis. In most genomes, the member of this family is encoded by a gene next to, and divergently transcribed from, the methyl coenzyme M reductase operon.</p>