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


Reviewed February 2016

What is the official name of the MMADHC gene?

The official name of this gene is “methylmalonic aciduria and homocystinuria, cblD type.”

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

What is the normal function of the MMADHC gene?

The MMADHC gene provides instructions for making a protein that helps convert vitamin B12 (also called cobalamin) into one of two molecules, adenosylcobalamin (AdoCbl) or methylcobalamin (MeCbl). AdoCbl is required for the normal function of an enzyme known as methylmalonyl CoA mutase. This enzyme helps break down certain protein building blocks (amino acids), fats (lipids), and cholesterol. AdoCbl is called a cofactor because it helps methylmalonyl CoA mutase carry out its function. MeCbl is also a cofactor, but for an enzyme known as methionine synthase. This enzyme converts the amino acid homocysteine to another amino acid, methionine. The body uses methionine to make proteins and other important compounds.

Research indicates that the MMADHC protein plays a role in one of the last steps in AdoCbl and MeCbl formation. Together with another protein called MMACHC (produced from the MMACHC gene), MMADHC transports vitamin B12 to regions of the cell in which each cofactor is needed: specialized structures that serve as energy-producing centers (the mitochondria), where AdoCbl functions, or the fluid inside the cell (the cytoplasm), where MeCbl functions. Additional chemical reactions then convert vitamin B12 into AdoCbl or MeCbl.

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

homocystinuria - caused by mutations in the MMADHC gene

At least seven mutations in the MMADHC gene cause a condition called homocystinuria, which is characterized by skeletal problems and intellectual disability. The MMADHC gene mutations that cause homocystinuria result in a protein that cannot transport vitamin B12 to the cytoplasm, where MeCbl is produced. The resulting shortage of MeCbl impairs methionine synthase's conversion of homocysteine to methionine. As a result, homocysteine builds up in the bloodstream and methionine is depleted. Some of the excess homocysteine is excreted in urine. Researchers have not determined how altered levels of homocysteine and methionine lead to the health problems associated with homocystinuria.

methylmalonic acidemia - caused by mutations in the MMADHC gene

At least three mutations in the MMADHC gene have been found to cause methylmalonic acidemia, a condition characterized by feeding difficulties, developmental delay, and long-term health problems. The MMADHC gene mutations that cause this condition result in a protein that cannot transport vitamin B12 to mitochondria for the production of AdoCbl. A lack of AdoCbl impairs the function of methylmalonyl CoA mutase. As a result, certain proteins and lipids are not broken down properly. This defect allows toxic compounds to build up in the body's organs and tissues, causing the signs and symptoms of methylmalonic acidemia.

methylmalonic acidemia with homocystinuria - caused by mutations in the MMADHC gene

At least three MMADHC gene mutations can cause methylmalonic acidemia with homocystinuria, cblD type, one form of a condition that has features of both of the two previously described conditions. People with this combined condition have developmental delay, eye defects, neurological problems, and blood abnormalities. The MMADHC gene mutations that cause this condition result in the production of a protein that cannot transport vitamin B12 to either the mitochondria or the cytoplasm, which disrupts production of both AdoCbl and MeCbl. Because both of these cofactors are missing, the enzymes that require them (methylmalonyl CoA mutase and methionine synthase) do not function normally. As a result, certain amino acids, lipids, and cholesterol are not broken down and homocysteine cannot be converted to methionine. This dual defect results in a buildup of toxic compounds as well as homocysteine, and a decrease in the production of methionine within the body. This combination of imbalances leads to the signs and symptoms of methylmalonic acidemia with homocystinuria.

Where is the MMADHC gene located?

Cytogenetic Location: 2q23.2

Molecular Location on chromosome 2: base pairs 149,569,633 to 149,587,816

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

The MMADHC gene is located on the long (q) arm of chromosome 2 at position 23.2.

The MMADHC gene is located on the long (q) arm of chromosome 2 at position 23.2.

More precisely, the MMADHC gene is located from base pair 149,569,633 to base pair 149,587,816 on chromosome 2.

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

Where can I find additional information about MMADHC?

You and your healthcare professional may find the following resources about MMADHC 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 MMADHC gene or gene products?

  • C2orf25
  • cblD
  • CL25022
  • methylmalonic aciduria and homocystinuria type D protein, mitochondrial
  • methylmalonic aciduria and homocystinuria type D protein, mitochondrial precursor
  • methylmalonic aciduria (cobalamin deficiency) cblD type, with homocystinuria

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

What glossary definitions help with understanding MMADHC?

acids ; aciduria ; amino acid ; cell ; cholesterol ; CoA ; cobalamin ; cofactor ; cytoplasm ; deficiency ; depleted ; developmental delay ; disability ; enzyme ; gene ; methionine ; mitochondria ; neurological ; precursor ; protein ; toxic ; vitamin B12

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


  • Coelho D, Suormala T, Stucki M, Lerner-Ellis JP, Rosenblatt DS, Newbold RF, Baumgartner MR, Fowler B. Gene identification for the cblD defect of vitamin B12 metabolism. N Engl J Med. 2008 Apr 3;358(14):1454-64. doi: 10.1056/NEJMoa072200. (
  • Froese DS, Kopec J, Fitzpatrick F, Schuller M, McCorvie TJ, Chalk R, Plessl T, Fettelschoss V, Fowler B, Baumgartner MR, Yue WW. Structural Insights into the MMACHC-MMADHC Protein Complex Involved in Vitamin B12 Trafficking. J Biol Chem. 2015 Dec 4;290(49):29167-77. doi: 10.1074/jbc.M115.683268. Epub 2015 Oct 19. (
  • Miousse IR, Watkins D, Coelho D, Rupar T, Crombez EA, Vilain E, Bernstein JA, Cowan T, Lee-Messer C, Enns GM, Fowler B, Rosenblatt DS. Clinical and molecular heterogeneity in patients with the cblD inborn error of cobalamin metabolism. J Pediatr. 2009 Apr;154(4):551-6. doi: 10.1016/j.jpeds.2008.10.043. Epub 2008 Dec 5. (
  • NCBI Gene (
  • Plesa M, Kim J, Paquette SG, Gagnon H, Ng-Thow-Hing C, Gibbs BF, Hancock MA, Rosenblatt DS, Coulton JW. Interaction between MMACHC and MMADHC, two human proteins participating in intracellular vitamin B₁₂ metabolism. Mol Genet Metab. 2011 Feb;102(2):139-48. doi: 10.1016/j.ymgme.2010.10.011. Epub 2010 Oct 21. (
  • Stucki M, Coelho D, Suormala T, Burda P, Fowler B, Baumgartner MR. Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism. Hum Mol Genet. 2012 Mar 15;21(6):1410-8. doi: 10.1093/hmg/ddr579. Epub 2011 Dec 8. (


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: February 2016
Published: February 8, 2016