Methylenetetrahydrofolate reductase (MTHFR) catalyses the reduction of methylenetetrahydrofolate to methyltetrahydrofolate, a cofactor for homocysteine methylation to methionine. MTHFR deficiency, an autosomal recessive disorder, results in homocysteinemia. Using degenerate oligonucleotides based on porcine peptide sequence data, we isolated a 90-bp cDNA by PCR from pig liver RNA. This cDNA was used to isolate a human cDNA, the predicted amino acid sequence of which shows strong homology to porcine MTHFR and to bacterial metF genes. The human gene has been localized to chromosome 1p36.3. Two mutations were identified in MTHFR-deficient patients: a missense mutation (Arg to Gln), in a residue conserved in bacterial enzymes, and a nonsense mutation (Arg to Ter).