A copper-based fluorescent probe promises to be a valuable tool for visualizing nitric oxide in living cells, according to its developers. Mi Hee Lim, Dong Xu, and Stephen J. Lippard of MIT hope to use such probes to detect intracellular levels of this key messenger in cellular-signaling networks (Nat. Chem. Biol., published online May 28, dx.doi.org/10.1038/nchembio794). Unlike other fluorescent sensors, their cell-permeable fluorescein-based Cu(II) probe (shown, X = anion) directly and immediately detects NO rather than oxidized NO derivatives, the authors note. NO reduces the complex to Cu(I) and triggers the irreversible nitrosation and release of the fluorescein ligand. The resulting bright visible-light emission can be detected by microscopy. Lippard's team shows that the probe, which is being commercialized by Strem Chemicals, can be used to image physiological levels of NO produced in living cells and can spatially distinguish which cells are producing NO in a mixed culture.