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CBS gene

cystathionine beta-synthase

Normal Function

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.

Health Conditions Related to Genetic Changes


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.

More About This Health Condition

Other Names for This Gene

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

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Catalog of Genes and Diseases from OMIM

Gene and Variant Databases


  • 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. doi: 10.1016/ Citation on PubMed
  • Kozich V, Sokolova J, Klatovska V, Krijt J, Janosik M, Jelinek 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. Citation on PubMed or Free article on PubMed Central
  • Kraus JP, Janosik 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. doi: 10.1002/(SICI)1098-1004(1999)13:53.0.CO;2-K. Citation on PubMed
  • 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. doi: 10.1016/s1570-9639(03)00048-7. Citation on PubMed
  • 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. doi: 10.1074/jbc.R400005200. Epub 2004 Apr 15. No abstract available. Citation on PubMed
  • 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. doi: 10.1002/humu.9214. Citation on PubMed

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.