APOE and Alzheimer disease: Microglial cells in AD were also enriched for genes involved in the regulation of neuron death, lipid-related clearance, response to oxidative stress (such as up-regulated LRRK2, APOE, and SNCA), response to extracellular stimulus, microglia activation (such as down-regulated AIF1), unfolded protein response, protein folding in endoplasmic reticulum (ER), endocytosis, and calcium ion retention (such as HSP90B1 and CALR) (Fig.1B-D).