MTHFR and homocystinuria: Homocystinuria due to methylenetetrahydrofolate reductase deficiency is caused by homozygous or compound heterozygous mutation in the MTHFR gene on chromosome 1p36. Methylenetetrahydrofolate reductase deficiency is a common inborn error of folate metabolism. The phenotypic spectrum ranges from severe neurologic deterioration and early death to asymptomatic adults. In the classic form, both thermostable and thermolabile enzyme variants have been identified