APOE and Alzheimer disease: In particular, fatigue has been associated with changes in cerebrospinal fluid (CSF) biomarkers of Alzheimer’s disease (AD) neuropathology [23], alterations of brain networks in mild cognitive impairment (MCI) [24], apolipoprotein E (ApoE) genotype [25], and amyloid deposition, functional and structural changes in the hippocampus [26–28].