Biotium also offers Light-On LysoView™ 555, a UV-activatable lysosome stain. In cells, the dye initially shows low fluorescence, but brief exposure to UV excitation from a mercury arc lamp activates bright red fluorescence localizing to lysosomes (Figure 3). The fluorescence fades after several minutes, but can be re-activated in the same cells multiple times by exposure to UV light, providing a novel tool for reversible induction of fluorescence in lysosomes.