Members of this family include a number of molybdopterin-containing oxidoreductases, tungsten formylmethanofuran dehydrogenase subunit d (FwdD) and molybdenum formylmethanofuran dehydrogenase subunit (FmdD); where a single domain constitutes almost the entire subunit. The formylmethanofuran dehydrogenase catalyses the first step in methane formation from CO2 in methanogenic archaea and has a molybdopterin dinucleotide cofactor [<cite idref="PUB00000170"/>].