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The CBS gene provides instructions for making an enzyme called cystathionine beta-synthase. This enzyme acts as a chemical pathway and is responsible for using vitamin B6 to convert 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 the urine. Additionally, other amino acids, including methionine, are produced in this pathway.

This pathway provides a vital source of the amino acid cysteine but is also the body's only mechanism for removing sulfur-containing amino acids when present in excess. 

Regardless of which pathway homocysteine is processed through it is considered harmful when present at high levels in the blood. Excessive homocysteine can damage endothelial cells which line the circulatory system and heart, leading to inflammation and increasing the risk of coronary artery disease.

The T allele is associated with an accumulation of ammonia resulting in allergies.