APOE and Parkinson disease: Consistent with this, activated microglia cluster 1 instead upregulates genes involved in antiviral defense, peptide metabolic processes, antigen processing and presentation, and protein synthesis, such as Cd74, Ifitm3, H2-Ab1, B2m, Ctss, and Apoe, while cluster 3 additionally expressed high levels of Tyrobp, Fcer1g, and Cxcl9. Notably, microglial expression of these genes has been implicated in aging or neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases and multiple sclerosis [14, 43, 44, 62, 63].