A range of mechanisms have been proposed for the link between elevated Hcy and AD and preclinical studies show that hyperhomocysteinemia, induced by genetic manipulation or by B-vitamin deficiency, causes known hallmarks of AD such as accumulation of amyloid-β peptide [133,134,135,136] and intensified tau protein hyperphosphorylation in the brain [137]. This evidence concerns the gene MAPT and Alzheimer disease.