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Genetics Home Reference: your guide to understanding genetic conditions     A service of the U.S. National Library of Medicine®


Reviewed July 2011

What is the official name of the MMAB gene?

The official name of this gene is “methylmalonic aciduria (cobalamin deficiency) cblB type.”

MMAB is the gene's official symbol. The MMAB gene is also known by other names, listed below.

What is the normal function of the MMAB gene?

The MMAB gene provides instructions for making an enzyme that is involved in the formation of a compound called adenosylcobalamin (AdoCbl). AdoCbl, which is derived from vitamin B12 (also known as cobalamin), is necessary for the normal function of another enzyme known as methylmalonyl CoA mutase. This enzyme helps break down certain proteins, fats (lipids), and cholesterol.

The MMAB enzyme is active in mitochondria, which are specialized structures inside cells that serve as energy-producing centers. Once vitamin B12 has been transported into mitochondria, the MMAB enzyme converts a form of the vitamin called cob(I)alamin to AdoCbl. Studies suggest that this enzyme may also deliver AdoCbl to methylmalonyl CoA mutase.

How are changes in the MMAB gene related to health conditions?

methylmalonic acidemia - caused by mutations in the MMAB gene

At least 25 mutations in the MMAB gene have been found to cause methylmalonic acidemia, a condition characterized by feeding difficulties, developmental delay, and long-term health problems. Some of these genetic changes delete or duplicate a small amount of genetic material in the MMAB gene. Other mutations change a single protein building block (amino acid) used to make the MMAB enzyme. Researchers believe that nearly all of these mutations lead to the production of a nonfunctional version of the enzyme. As a result, AdoCbl cannot be made properly. A lack of AdoCbl impairs the function of methylmalonyl CoA mutase, which results in the incomplete breakdown of certain proteins and lipids. This defect allows toxic compounds to build up in the body's organs and tissues, causing the signs and symptoms of methylmalonic acidemia.

Where is the MMAB gene located?

Cytogenetic Location: 12q24

Molecular Location on chromosome 12: base pairs 109,553,715 to 109,573,553

(Homo sapiens Annotation Release 107, GRCh38.p2) (NCBI (

The MMAB gene is located on the long (q) arm of chromosome 12 at position 24.

The MMAB gene is located on the long (q) arm of chromosome 12 at position 24.

More precisely, the MMAB gene is located from base pair 109,553,715 to base pair 109,573,553 on chromosome 12.

See How do geneticists indicate the location of a gene? ( in the Handbook.

Where can I find additional information about MMAB?

You and your healthcare professional may find the following resources about MMAB helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the MMAB gene or gene products?

  • ATP:Cob(I)alamin Adenosyltransferase
  • ATR
  • cblB
  • CFAP23
  • cob(I)alamin adenosyltransferase
  • methylmalonic aciduria (cobalamin deficiency) type B

See How are genetic conditions and genes named? ( in the Handbook.

What glossary definitions help with understanding MMAB?

aciduria ; amino acid ; ATP ; breakdown ; cholesterol ; CoA ; cobalamin ; cofactor ; compound ; deficiency ; developmental delay ; enzyme ; gene ; mitochondria ; protein ; toxic ; vitamin B12

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.


  • Chandler RJ, Venditti CP. Genetic and genomic systems to study methylmalonic acidemia. Mol Genet Metab. 2005 Sep-Oct;86(1-2):34-43. Epub 2005 Sep 22. Review. (
  • Dobson CM, Wai T, Leclerc D, Kadir H, Narang M, Lerner-Ellis JP, Hudson TJ, Rosenblatt DS, Gravel RA. Identification of the gene responsible for the cblB complementation group of vitamin B12-dependent methylmalonic aciduria. Hum Mol Genet. 2002 Dec 15;11(26):3361-9. (
  • Gene Review: Isolated Methylmalonic Acidemia (
  • Hörster F, Baumgartner MR, Viardot C, Suormala T, Burgard P, Fowler B, Hoffmann GF, Garbade SF, Kölker S, Baumgartner ER. Long-term outcome in methylmalonic acidurias is influenced by the underlying defect (mut0, mut-, cblA, cblB). Pediatr Res. 2007 Aug;62(2):225-30. (
  • Leal NA, Olteanu H, Banerjee R, Bobik TA. Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase. J Biol Chem. 2004 Nov 12;279(46):47536-42. Epub 2004 Aug 30. (
  • Leal NA, Park SD, Kima PE, Bobik TA. Identification of the human and bovine ATP:Cob(I)alamin adenosyltransferase cDNAs based on complementation of a bacterial mutant. J Biol Chem. 2003 Mar 14;278(11):9227-34. Epub 2003 Jan 3. (
  • Lerner-Ellis JP, Gradinger AB, Watkins D, Tirone JC, Villeneuve A, Dobson CM, Montpetit A, Lepage P, Gravel RA, Rosenblatt DS. Mutation and biochemical analysis of patients belonging to the cblB complementation class of vitamin B12-dependent methylmalonic aciduria. Mol Genet Metab. 2006 Mar;87(3):219-25. Epub 2006 Jan 10. (
  • NCBI Gene (
  • Zhang J, Dobson CM, Wu X, Lerner-Ellis J, Rosenblatt DS, Gravel RA. Impact of cblB mutations on the function of ATP:cob(I)alamin adenosyltransferase in disorders of vitamin B12 metabolism. Mol Genet Metab. 2006 Apr;87(4):315-22. Epub 2006 Jan 24. (


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? ( in the Handbook.

Reviewed: July 2011
Published: February 8, 2016