FM4-64 is a red fluorescent dye commonly used to study synaptic activity at synapses and neuromuscular junctions by staining synaptic vesicles. It is water-soluble, non-toxic, and initially nonfluorescent in aqueous medium. However, after embedding into the outer leaflet of cell membranes, it becomes highly fluorescent. During neurotransmitter release, it internalizes within recycled synaptic vesicles, staining nerve terminals.
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