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CBS gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

CBS gene

The CBS gene provides instructions for making the enzyme cystathionine beta-synthase, which is critical for processing amino acids and maintaining healthy metabolic function.

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 a building block of proteins (amino acid) called homocysteine and serine to a molecule called cystathionine. 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

Homocystinuria

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. Common mutations include:

  • Ile278Thr (I278T): Substitutes threonine for isoleucine at position 278.
  • Gly307Ser (G307S): Replaces glycine with serine at position 307 (the most frequent cause of homocystinuria in the Irish population).

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 specific signs and symptoms of homocystinuria.

Other Names for This Gene

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

Frequently Asked Questions

Newborn screening programs in many regions include a test for elevated levels of homocysteine in the blood. If the initial screen is positive, it is confirmed through follow-up blood and urine tests that measure specific amino acid levels. Genetic testing of the CBS gene can then be performed to identify the specific mutation, which helps determine the severity of the condition and the likelihood of response to vitamin B6 therapy.
The G307S mutation is particularly significant because it is the most common cause of homocystinuria in the Irish population and is frequently associated with a partial response to pyridoxine (vitamin B6) supplementation. Identifying this specific variant allows clinicians to initiate a trial of high-dose vitamin B6 earlier, as patients with this mutation often have better metabolic control with this therapy compared to those with other severe mutations.
Management requires a diet low in methionine, an amino acid that is a precursor to homocysteine. Patients must avoid high-protein foods such as meat, fish, eggs, and dairy, and often require a specialized medical formula to provide essential nutrients without excess methionine. Strict adherence is critical because even small dietary indiscretions can cause a rapid spike in homocysteine levels, increasing the risk of clotting and neurological complications.
Yes, chronic elevation of homocysteine is a major risk factor for thromboembolic events, including deep vein thrombosis, pulmonary embolism, and stroke. Adults who were not diagnosed in childhood may present with these acute vascular events as their first sign of the disorder. A history of unexplained blood clots at a young age should prompt immediate testing for homocysteine levels and potential CBS gene analysis.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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