The polymorphism 677C>T (NM_005957.4:c.665C>T/rs1801133" genes_norm="4524">p.Ala222Val, rs1801133:C>T) in methylenetetrahydrofolate reductase (MTHFR) results in mild enzymatic deficiency and increased risk for several complex traits including adverse reproductive outcomes, birth defects, and heart disease.