This model describes a clade of ABC porter genes with relatively weak homology compared to its neighbor clades, the molybdate (TIGR02141) and sulfate (TIGR00969) porters. Neighbor-Joining, PAM-distance phylogenetic trees support the separation of the clades in this way. Included in this group is a gene designated NifC in Clostridium pasturianum. It would be reasonable to presume that NifC acts as a molybdate porter since the most common form of nitrogenase is a molybdoenzyme. Several other sequences falling within the scope of this model are annotated as molybdate porters and one, from Halobacterium, is annotated as a sulfate porter. There is presently no experimental evidence to support annotations with this degree of specificity.