Several population studies have shown an association of the the gene for methylenetetrahydrofolate reductase (MTHFR), an enzyme required for folate metabolism and the generation of methyl groups [19], [20], with global changes in DNA methylation [21]–[23] and depressed mood and depressive disorders in non-pregnant populations [24]–[28]. This evidence concerns the gene MTHFR and depressive disorder.