The MopB_FmdB-FwdB CD contains the molybdenum/tungsten formylmethanofuran dehydrogenases, subunit B (FmdB/FwdB), and other related proteins. Formylmethanofuran dehydrogenase catalyzes the first step in methane formation from CO2 in methanogenic archaea and some eubacteria. Members of this CD belong to the molybdopterin_binding (MopB) superfamily of proteins.