Although the mechanisms are yet unknown, preliminary studies showed that ApoE was expressed at high levels in various types of cells in the olfactory epithelium and its underlying lamina propria, and that ApoE might play a critical role in olfactory nerve regeneration in mice.39,40 Furthermore, experimental studies suggest that in contrast to the ApoE ε2 and ε3 alleles, the ApoE ε4 fails to promote neurite outgrowth in olfactory epithelium cultures, which may lead to olfactory dysfunction in AD.41 Here, APOE is linked to Alzheimer disease.