In accord with this notion, single-nucleotide polymorphisms (SNPs) in genes for methionine synthase (MTR), methionine synthase reductase (MTRR), transcobalamin (TCN2) and 5,10-methylenetetrahydrofolate reductase (MTHFR), which limit their respective activities, are associated with increased risk of autism [57,84] and schizophrenia [83,85,86], as well as major depression and bipolar disorder [85,86], Parkinson’s disease [87] and Alzheimer’s disease [86,88]. Here, TCN2 is linked to major depressive disorder.