MTR and mental disorder: The risk of development of mental disorders, such as autism and schizophrenia, is increased if patients have mutations in the genes associated with vitamin B12 transport and one-carbon metabolism: methionine synthase (MTR), methionine synthase reductase (MTRR), transcobalamin (TCN2), and 5,10-methylenetetrahydrofolate reductase (MTHFR), reducing their activity [29].