Fuso et al. [75], have shown that the elevated concentrations of Hcy in AD may lower the level of S-adenosylmethionine (SAM), essential for DNA methylation, thus may affect activity of presenilin 1 (PS1) and beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), responsible for Aβ synthesis, associated with severity of the disease. This evidence concerns the gene PSEN1 and Alzheimer disease.