APOE and major depressive disorder: The cause of FAD is mutations in PSEN1 and PSEN2 genes, but SAD is a multifactorial pathology; some of the risk factors are allelic variations of apolipoprotein E (ApoE), metal exposure, traumatic brain injury (TBI), cardiovascular disease, depression, stress, sleep disorders, and age, which is one of the main risk factors [4,5,6].