Bifunctional mitochondrial folate-interconverting enzyme that has both NAD/NADP-dependent methylenetetrahydrofolate dehydrogenase and methenyltetrahydrofolate cyclohydrolase activities. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NAD(+) = 5,10-methenyltetrahydrofolate + NADH 5,10-methenyltetrahydrofolate + H2O = (6S)-10-formyltetrahydrofolate + H(+) (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NADP(+) = 5,10-methenyltetrahydrofolate + NADPH One-carbon metabolism; tetrahydrofolate interconversion. Widely expressed. Expressed at all embryonic days examined in the neural tube and the forebrain, midbrain, and hindbrain. Also detected in the branchial arches and limb buds, particularly along the progress zone. Expression is low at E8.5 but increases at E10.5. Belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family.