MTHFR and Alzheimer disease: MTHFR functions in the folate/methionine/homocysteine pathway are expressed both peripherally and centrally, and the MTHFR677C >T variant causes an increase in homocysteine increasing the risk for a variety of diseases including cardiovascular and cerebrovascular-related disorders and AD (Liu et al., 2010; Liew and Gupta, 2015; Rai, 2017).