MTHFR (5,10-methylenetetrahydrofolate reductase) is an enzyme involved in the metabolism of folate. MTHFR catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the major circulating form of folate. In turn, 5-methyltetrahydrofolate is involved in the conversion of homocysteine to methionine. MTHFR has an important role in maintaining folate and methionine levels, as well as helping to keep circulating homocysteine levels low. MTHFR is also involved in the methylation pathway, which has multiple, wide-ranging roles in the body, including regulation of gene expression and enzymatic activities.