Light-On LysoView™ 555 is a red fluorogenic lysosome dye with pH-dependent fluorescence. Light-On LysoView™ dye is unique among commercially available lysosomotropic dyes in that its fluorescence in cells is activated by exposure to UV excitation. In solution, the dye shows pH-dependent fluorescence that does not require UV activation. In cells, the dye initially shows low fluorescence, but brief exposure to UV excitation from a mercury arc lamp through a DAPI filter activates bright red fluorescence localizing to lysosomes. We hypothesize that the dye assumes a non-fluorescent structure that can be switched to a fluorescent structure by UV excitation. Mercury arc lamp excitation of the dye using other filter sets (FITC, rhodamine) or dye excitation with a 405 nm laser does not activate fluorescence. Lysosomal fluorescence fades several minutes after UV exposure, but can be re-activated in the same cells multiple times by exposure to UV light . Therefore the dye provides a novel tool for UV-activated, reversible fluorescence imaging of lysosomes. See the product protocol under the downloads tab for more information.