Interestingly, many defective DNA repair genes in AD are not methylated in their promoters 196, 197, although DNA damage response factors, such as p53, poly(ADP-ribose) polymerase 1 (PARP-1) and Growth arrest and DNA damage-inducible alpha (GADD45a), have been shown to stimulate DNA methylation 198, 199. This evidence concerns the gene PARP1 and Alzheimer disease.