Importantly, we find cell type-specific enrichments of age associations with genes nominated by Alzheimer’s disease and Parkinson’s disease genome-wide association studies (GWAS), such as apolipoprotein E (APOE), and leucine-rich repeat kinase 2 (LRRK2) in microglia that are independent of overall expression levels across cell types. The gene discussed is APOE; the disease is early-onset autosomal dominant Alzheimer disease.