The methylmalonic acidurias are metabolic disorders resulting from deficient methylmalonyl-CoA mutase activity, a vitamin B(12)-dependent enzyme. We have cloned the gene for the cblB complementation group caused by deficient activity of a cob(I)alamin adenosyltransferase. This was accomplished by searching bacterial genomes for genes in close proximity to the methylmalonyl-CoA mutase gene that might encode a protein with the properties of an adenosyltransferase. A candidate was identified in the Archaeoglobus fulgidus genome and was used to probe the human genome database. It yielded a gene on chromosome 12q24 that encodes a predicted protein of 250 amino acids with 45% similarity to PduO in Salmonella enterica, a characterized cob(I)alamin adenosyltransferase. A northern blot revealed an RNA species of 1.1 kb predominating in liver and skeletal muscle. The gene was evaluated for deleterious mutations in cblB patient cell lines. Several mutations were identified including a 5 bp deletion (5del572gggcc576), two splice site mutations (IVS2-1G>T, IVS3-1G>A), andt several point mutations (A135T, R186W, R191W and E193K). Two additional amino acid substitutions (R19Q and M239K) were found in several patient cell lines but were found to be common polymorphisms (36% and 46%) in control alleles. The R186W mutation, which we suggest is disease-linked, is present in four of the six patient cell lines examined (homoallelic in two) and in 4 of 240 alleles in control samples. These data confirm that the identified gene, MMAB, corresponds to the cblB complementation group and has the appearance of a cob(I)alamin adenosyltransferase, as predicted from biochemical data.