PSEN1 and Alzheimer disease: During the past decades, studies suggested that multiple factors [6] including biological factors (e.g., aging, gender, body weight, etc.), environmental factors (e.g., lifestyle, toxins, brain injury, etc.), and genetic factors (e.g., APP, PS1, and PS2 genetic mutation in familial AD and susceptibility genetic polymorphisms in sporadic cases) contribute to the pathogenesis of AD.