They seal epithelial cells and, under certain conditions, allow the passage of ions and water: Tight junctions form a paracellular barrier in tissues and their dysfunction is associated with diseases. Although their molecular components have been known since the 1990s, it is not apparent how the 26 proteins called claudins are organized. Scientists from the Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) have now gained deep insights into the structure of tight junctions, using super-resolution stimulated emission depletion (STED) microscopy. It is the first time that the basic mechanism underlying all epithelial barrier properties has been described.