Editor—Recent studies have shown that periconceptual folic acid supplementation reduces a woman’s risk of having a baby with neural tube defects (NTD). Mothers of infants with NTD have increased homocysteine activities. People with a thermolabile form of the enzyme 5,10 methylenetetrahydrofolate reductase (MTHFR) have reduced enzyme activity and increased plasma homocysteine which can be lowered by supplemental folic acid. Thermolability of the enzyme is caused by a common mutation (677→CT) in the MTHFR gene. In different populations the 677→CT mutation has been implicated in susceptibility to NTD.1 We studied the 677→CT mutation as a risk factor for spine bifida …