[citation needed] Signs and symptoms of homocystinuria that may be seen include the following: It is usually caused by the deficiency of the enzyme cystathionine beta synthase,[3] mutations of other related enzymes such as methionine synthase,[3] or the deficiency of folic acid, vitamin B12 and/or pyridoxine (vitamin B6).

In the first instance, plasma or urine amino acid analysis will frequently show an elevation of methionine and the presence of homocysteine.

To this end it is prudent to perform total homocyst(e)ine analysis in which all disulphide bonds are subject to reduction prior to analysis, traditionally by HPLC after derivatisation with a fluorescent agent, thus giving a true reflection of the quantity of homocysteine in a plasma sample.

[9] No specific cure has been discovered for homocystinuria; however, many people are treated using high doses of vitamin B6 (also known as pyridoxine).

[citation needed] For the subset of patients whose homocystinuria is caused by cystathionine beta-synthase deficiency (i.e. those that do not respond to B6 supplementation), low-protein food is recommended for this disorder, which requires food products low in particular types of amino acids (e.g., methionine) in addition to supplementation.

It is known that before the age of 30, almost one quarter of patients die as a result of thrombotic complications (e.g., heart attack).

[citation needed] One theory suggests that Akhenaten, a pharaoh of the eighteenth dynasty of Egypt, may have had homocystinuria.