Homocystinuria is an autosomal recessive metabolic disease that leads to elevated levels of homocysteine due to functional defects in the enzyme cystathionine-beta-synthase (CBS). Globally, incidence is low, but appears higher in clinical samples from some regions, and effects on many patients, involving the skeletal, nervous and vascular systems, can be pronounced. Furthermore, some mutated patients exhibit no symptoms, and those who are heterozygous for a CBS mutation may be at increased risk for vascular disease.