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. The gene discussed is NRGN; the disease is Alzheimer disease.