Homocystinuria (HCY)



Homocystinuria (HCY) is a condition in which the body is unable to break down certain proteins. Individuals with HCY cannot process certain amino acids, small molecules that make up proteins. These amino acids build up in the body and can cause serious health problems. However, if detected early and treatment is begun, individuals with homocystinuria can often lead healthy lives. Babies with HCY are usually healthy at birth, but will typically begin to show signs of HCY between ages 1 and 3. The signs are highly variable among individuals with HCY and can include poor growth, trouble gaining weight, developmental delays, behavioral difficulties, weak muscle tone (hypotonia), pale skin and hair, and seizures (epilepsy). Many of these signs may occur when a baby eats foods the body cannot break down.

Condition Type:

Core Conditions


Homocystinuria (HCY) affects about 1 out of every 200,000 to 300,000 babies born in the United States. The condition is more common in certain ethnic groups and populations.

More Information for Parents:
Also known as:
  • CBS deficiency
  • HCY
  • HCU
  • Cystathionine beta-synthase deficiency
  • Homocystinemia

Core Conditions