APOE and neurodegenerative disease: The etiologies that lead to the development of brain abnormalities are often multifactorial, including aging, environmental factors, chronic stress, traumatic brain injury, and gene mutations such as those identified in amyloid precursor protein (APP), presenilin-1 and -2, or apolipoprotein E (ApoE), which have all been associated with the development of neurodegenerative diseases [4].