In recent years, several techniques have been developed to beat the diffraction limit for optical microscopy. Two independent teams of researchers have now developed another diffraction-beating technique, based on a workhorse of modern cell biology—a fluorescing molecule, or fluorophore, that can be made to attach to a variety of targets in a cell. In the new technique, the fluorophores are photo-switchable; when dimly and briefly illuminated at an appropriate wavelength, only a few in the field of view are activated at any one time. The brightest ones can then be localized to within a few nanometers. By repeatedly imaging the same area and adding up many such sparse images, researchers build a composite that displays the entire field of view with near-molecular resolution. A group led by Eric Betzig (Howard Hughes Medical Institute [HHMI], Janelia Farm Research Campus, in Ashburn, Virginia) and Harald Hess (NuQuest Research LLC in La Jolla,...

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