MTHFR and Alzheimer disease: It is therefore not surprising that studies concentrating on a single polymorphism and its association with AD failed to show a consistent picture: A number of studies did not detect a difference in the frequency of the MTHFR C677T genotype in AD patients and controls [113,114,115,116,117,118,119,120,121,122,123,124,125,126], which is probably only partially due to the small sample sizes.