<unk>  is the most common genetic cause of elevated serum levels of homocysteine (hyperhomocysteinemia). It is caused by genetic defects in MTHFR, which is an important enzyme in the methyl cycle. Common variants of MTHFR deficiency are asymptomatic and have only minor effects on disease risk. Severe variants (from nonsense mutations) are vanishingly rare.
Methylenetetrahydrofolate reductase deficiency
Dihydrofolate reductase deficiency
Adenine phosphoribosyltransferase deficiency
Guanidinoacetate methyltransferase deficiency
5,10-methenyltetrahydrofolate synthetase deficiency
Purine nucleoside phosphorylase deficiency
Dihydropyrimidine dehydrogenase deficiency
Adenosine deaminase 2 deficiency