The impact of brain region and ageing on microglia is unknown. Grabert et al. analysed transcriptomes of microglia from different brain regions (including the hippocampus, cortex and cerebellum) in 4-, 12- and 22-month-old mice using various clustering techniques. They found that differences in the expression of bioenergetics- and immune-related genes best differentiated microglia from different regions at 4 months, and that changes in expression with age were also region-specific.