Nevertheless, one large case-control study found that there was no difference in the blood DNA methylation levels of PSEN1 or BACE1, both codes for proteins directly involved in the APP cleavage, between patients with AD and the controls, leading to the formation of the β-amyloid (Aβ) (Tannorella et al., 2015). Here, PSEN1 is linked to Alzheimer disease.