Emitting at the far-red end of the visible light spectrum, a DNA stain could enable imaging of live cells over the course of a day or more. The stain has three key advantages over existing fluorescent tags used in live-cell imaging, according to its developers at the Swiss Federal Institute of Technology in Lausanne (EPFL). Live-cell imaging can help scientists track key biological processes such as cell division. But the fluorescent stains that light up DNA within cells are toxic, or require blue or UV light that can damage cells. In addition, many DNA stains are not compatible with superresolution microscopy techniques, which capture higher-resolution images of cells than regular optical microscopes. The new stain, called SiR-Hoechst, is superresolution-compatible, displays minimal toxicity and emits red light with very little noise once bound to a part of the DNA helix known as the minor groove. Mitosis of a live HeLa cell stained with SiR-Hoechst, whose chemical structure is superimposed. Courtesy of Kai Johnsson/EPFL. "The introduction of SiR-Hoechst brings bioimaging closer to one of its main goals: observing the wonders of nature without disturbing them," said EPFL professor Kai Johnsson. SiR-Hoechst incorporates two molecules. The first is a fluorescent molecule (silicon rhodamine, or SiR) that works in the far-red spectrum and was developed previously in Johnsson's lab. The second is the well-known DNA stain Hoechst (bisbenzimide). Because SiR-Hoechst works with far-red light, there is little risk of photodamage to cells. In addition, the light it emits can be easily distinguished from any background fluorescence of living cells. The stain lasts for more than 24 hours, allowing biologists to identify individual cells in tissue or culture, or to track delicate processes such as cell division in real time. And because all cells possess DNA, the probe can be used across numerous species and types of cells and tissues. The researchers are now preparing to commercialize SiR-Hoechst through the EPFL spinoff company Spirochrome SA. The company already provides fluorescent probes for imaging the cytoskeletons of living cells. The research was published in Nature Communications (doi: 10.1038/ncomms9497).