Homocystinuria or HCU is an inherited disorder of the metabolism of the amino acid methionine due to a deficiency of cystathionine beta synthase or methionine synthase. It is an inherited autosomal recessive trait, which means a child needs to inherit a copy of the defective gene from both parents to be affected. Symptoms of homocystinuria can also be caused by a deficiency of vitamins B6, B12, or folate.