To observe content mixing between virus and vesicle compartments, typically, a water-soluble dye is encapsulated at self-quenching concentration within the virus or target vesicle interior which turns on in fluorescence due to dilution upon fusion . These assays have been popular because content label dyes like calcein and sulforhodamine B (SRB) are relatively inexpensive (an important practical consideration since only a small fraction of dye gets encapsulated during preparation of the target vesicles). However, they suffer from a few drawbacks. First, a very high concentration (tens of mM) of the content label dye needs to be encapsulated in the target vesicle lumen. It has been shown that dyes like SRB can strongly associate with lipid membranes and therefore it can be expected that such dyes may perturb the mechanical properties of the target vesicle membranes and therefore influence the fusion behavior. Second, content loss events due to vesicle bursting complicate the analysis of these assays . Finally, content mixing assays based on dequenching of soluble dyes have to rely on indirect evidence and do not detect the viral macromolecular content (genome) directly. Therefore, a probe which enables direct visualization of viral RNA in infectious particles without potentially perturbing the target vesicle membranes would be highly desirable.