A new imaging agent enters live cells readily and makes it possible to visualize DNA, including special DNA conformations such as quadruplexes (Nat. Chem., DOI: 10.1038/nchem.406). Currently, luminescent, cell-membrane-permeable stains can mark DNA and make it viewable by fluorescence microscopy, but many of these dyes are poorly soluble in water, highly toxic, prone to photobleaching (loss of fluorescent activity), and subject to background interferences. What’s more, their emission is activated only by UV light, which can damage cells. DNA markers made from metal ion complexes are activated in the safer visible region and are less prone to background interference but are generally cell-membrane-impermeable. Now, Giuseppe Battaglia, Jim A. Thomas, and coworkers of the University of Sheffield, in England, have identified a ruthenium(II) polypyridyl complex that is actively transported across cell membranes into living cells and acts as a structure-sensitive DNA probe, detectable by luminescence microscopy or transition electron microscopy. It has multiple emission peaks and shines at a different frequency when bound to quadruplex instead of conventional duplex DNA, suggesting that it might find use as a quadruplex-DNA imaging tool.