This composite image shows axons of rat primary cortical neurons expressing mitochondrially-targeted DsRed2 (red) and GFP-tagged endoplasmic reticulum (ER) protein Sec61beta (green). At 14 days in vitro, neurons were exposed to 5-ethynyl-2’-deoxyuridine (white), which was incorporated into replicating mitochondrial DNA at interaction sites between mitochondria and ER in distal axons, away from the cell body. Images were acquired using super-resolution stimulated emission depletion (STED) microscopy. For more information, see the article by Van Laar et al. (pages 7505–7515).