Homocystinuria is disorder of the metabolism. It causes low levels of a specific enzyme. These enzymes normally help to break down the amino acids methionine and homocysteine. This causes the buildup of these amino acids.

The most common form of homocystinuria is caused by a deficiency of the enzyme cystathionine beta-synthase. Homocystinuria is a genetic disorder. The faulty genes are inherited from the parents. Both parents must have the defective gene in order for the child to develop the condition.