Importantly, it was found that although the AD risk gene apolipoprotein E (APOE) was repressed in astrocyte subpopulations and in AD-related oligodendrocyte progenitor cells, it was upregulated in an AD-specific microglial subpopulations within the EC, suggesting that these specialized cells contribute to disease susceptibility of the EC during AD. This evidence concerns the gene APOE and Alzheimer disease.