The tubulin network of PtK2 cells can be stained with the photoactivatable fluorophore 4a. Illumination at an activating wavelength () converts 4a into 4b, which can be irradiated at an exciting wavelength () to emit light in the form of fluorescence and eventually bleach. Reiterative sequences of activation and excitation events can be exploited to localize sequentially multiple fluorescent labels and reconstruct an image (a) with a significant improvement in spatial resolution relative to a conventional diffraction-limited counterpart (b). The scale bar in (a) corresponds to 2 μm (reproduced with permission from [98]).