This subfamily is comprised of systems which are found in operons encoding genes involved in cobalamin (Vitamin B-12) biosynthesis. Cobalamin chelates a cobalt ion in its center. CBU systems contain three proteins, CbiQ, CbiO and CbiN which were proposed to constitute a cobalt uptake system because CbiN and CbiQ are integral membrane proteins and CbiQ is an ABC ATPase ( Pubmed : 8501034 ) . However, direct evidence supporting this idea is lacking. Mutants affecting cobalamin biosynthesis were never found in cbiNOQ genes. The CbiQ proteins are related to ABC of the MET-ISVH families. No substrate binding protein has been identified in the cbi operons and the exact function of the cbiNOQ genes remains unknown. The genomes of several eubacteria and archaea contain ORFs orthologous to the cbiNOQ genes, which are located close to ORFs homologous to the cobalamin biosynthetic gene cbiM.