Classical homocystinuria (HCU), also known as homocystinuria due to CBS deficiency, is a rare genetic metabolic disorder caused by a deficiency in the enzyme cystathionine beta synthase (CBS). Without enough CBS, the amino acids homocysteine (Hcy) and methionine build up in the blood, while causing a deficiency in cysteine. Normal levels of these amino acids are essential for proper growth and development.

Toxic levels of homocysteine can lead to life-threatening thrombotic events such as stroke and heart attacks. Other symptoms of the disease include nearsightedness, dislocation of the lens of the eye, and developmental delay.

Current treatment options include significant dietary restrictions and supplemental use of vitamin B6 and betaine. For many individuals with classical HCU, homocysteine levels are unresponsive to these treatments.

The disorder is inherited in an autosomal recessive genetic manner, meaning that a child is affected when each parent passes on one copy of the mutated gene.

Learn about our development efforts for classical HCU.

Resources: NIH Genetics Home Reference, National Organization for Rare Disorders.


Find resources, community and support through the patient advocacy organizations below.