MTHFR and schizophrenia: The homozygous (TT) polymorphism at position 677 of the methylene tetrahydrofolate reductase (MTHFR) gene has a well-known association with developmental neurological defects, such as spina bifida [25], and the role of this gene polymorphism in schizophrenia has been established by meta-analysis [26,27].