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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 CBS gene?

The official name of this gene is “cystathionine-beta-synthase.”

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

What is the normal function of the CBS gene?

The CBS gene provides instructions for making an enzyme called cystathionine beta-synthase. This enzyme acts in a chemical pathway and is responsible for using vitamin B6 to convert building block of proteins (amino acid) called homocysteine and serine to a molecule called cytathionine. Another enzyme then converts cystathionine to the amino acid cysteine, which is used to build proteins or is broken down and excreted in urine. Additionally, other amino acids, including methionine, are produced in this pathway.

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

homocystinuria - caused by mutations in the CBS gene

More than 150 mutations that cause homocystinuria have been identified in the CBS gene. Most of these mutations change single amino acids in cystathionine beta-synthase. The most common mutation substitutes the amino acid threonine for the amino acid isoleucine at position 278 in the enzyme (written as Ile278Thr or I278T). Another common mutation, which is the most frequent cause of homocystinuria in the Irish population, replaces the amino acid glycine with the amino acid serine at position 307 (written as Gly307Ser or G307S). These mutations disrupt the normal function of cystathionine beta-synthase. As a result, homocysteine and other potentially toxic compounds build up in the blood, and homocysteine is excreted in urine. Researchers have not determined how excess homocysteine leads to the signs and symptoms of homocystinuria.

Where is the CBS gene located?

Cytogenetic Location: 21q22.3

Molecular Location on chromosome 21: base pairs 43,053,191 to 43,076,868

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

The CBS gene is located on the long (q) arm of chromosome 21 at position 22.3.

The CBS gene is located on the long (q) arm of chromosome 21 at position 22.3.

More precisely, the CBS gene is located from base pair 43,053,191 to base pair 43,076,868 on chromosome 21.

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

Where can I find additional information about CBS?

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

  • beta-thionase
  • HIP4
  • methylcysteine synthase
  • serine sulfhydrase

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

What glossary definitions help with understanding CBS?

acids ; amino acid ; cysteine ; enzyme ; gene ; glycine ; isoleucine ; methionine ; molecule ; mutation ; population ; serine ; threonine ; toxic

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


  • Banerjee R, Zou CG. Redox regulation and reaction mechanism of human cystathionine-beta-synthase: a PLP-dependent hemesensor protein. Arch Biochem Biophys. 2005 Jan 1;433(1):144-56. Review. (
  • Kozich V, Sokolová J, Klatovská V, Krijt J, Janosík M, Jelínek K, Kraus JP. Cystathionine beta-synthase mutations: effect of mutation topology on folding and activity. Hum Mutat. 2010 Jul;31(7):809-19. doi: 10.1002/humu.21273. (
  • Kraus JP, Janosík M, Kozich V, Mandell R, Shih V, Sperandeo MP, Sebastio G, de Franchis R, Andria G, Kluijtmans LA, Blom H, Boers GH, Gordon RB, Kamoun P, Tsai MY, Kruger WD, Koch HG, Ohura T, Gaustadnes M. Cystathionine beta-synthase mutations in homocystinuria. Hum Mutat. 1999;13(5):362-75. Review. (
  • Meier M, Oliveriusova J, Kraus JP, Burkhard P. Structural insights into mutations of cystathionine beta-synthase. Biochim Biophys Acta. 2003 Apr 11;1647(1-2):206-13. Review. (
  • Miles EW, Kraus JP. Cystathionine beta-synthase: structure, function, regulation, and location of homocystinuria-causing mutations. J Biol Chem. 2004 Jul 16;279(29):29871-4. Epub 2004 Apr 15. Review. (
  • Moat SJ, Bao L, Fowler B, Bonham JR, Walter JH, Kraus JP. The molecular basis of cystathionine beta-synthase (CBS) deficiency in UK and US patients with homocystinuria. Hum Mutat. 2004 Feb;23(2):206. (
  • NCBI Gene (


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