APOE and Alzheimer disease: Interestingly, higher CSF Ng concentrations were observed in the MCI ε4+ group than MCI ε4− group, but there was no similar finding between CN ε4− and CN ε4+ and between AD ε4− and AD ε4+, suggesting that CSF Ng may be an early biomarker of AD-related synaptic degeneration (Portelius et al., 2015), and suggesting the roles of CSF Ng in the pathophysiology of MCI may be related to APOE ε4 status.