Homocystinuria is an autosomal recessive disorder of methionine metabolism that produces an abnormal accumulation of homocystine, homocysteine-cysteine complex, and other metabolites in the blood and urine.

The incidence of homocystinuria is variable, with reports between 1:50,000 to 1:200,000.

Homocystinuria is an autosomal recessive inherited defect in either the transsulfuration pathway or methylation pathway, producing deficiencies of cystathionine beta-synthase enzyme, defective methylcobalamin synthesis, or an abnormality of methylene tetrahydrofolate reductase.