This entry represents a family of proteins which have been proposed to act as cobalt transporters acting in concert with vitamin B12 biosynthesis systems. Evidence for this assignment includes 1) prediction of five transmembrane segments, 2) positional gene linkage with known B12 biosynthesis genes, 3) upstream proximity of B12 transcriptional regulatory sites, 4) the absence of other known cobalt import systems and 5) the obligate co-localisation with a small protein (CbtB) having a single additional transmembrane segment and a C-terminal histidine-rich motif likely to be a metal-binding site.