The T allele of the MTHFR c.677C > T gene may be associated folate and vitamin B12 deficiency and hyperhomocysteinemia causing impaired one-carbon transfer (methylation) reactions, which are necessary for the production of monoamine neurotransmitters, phospholipids, and nucleotides [42]. The gene discussed is MTHFR; the disease is vitamin B12 deficiency.