For example, genotype × genotype analyses involving COMT Val158Met with the methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism have shown strong interactive effects associated with elevated total plasma homocysteine in a case control study of elders with and without dementia[22] as well as on dopamine signaling[23], executive function[24], and cognition[25] in persons with schizophrenia. Here, MTHFR is linked to dementia.