The requirement of HIV-1 and HTLV-1 to export incompletely spliced mRNAs has necessitated the evolution of Rev and Rex, respectively, to overcome host cellular mechanism that block nuclear-cytoplasmic export of incompletely processed mRNAs. Evaluating the function of these viral factors can be done at multiple levels: examining the functional consequence of Rev/Rex on viral gene expression, monitoring the movement of these proteins between the nucleus and cytoplasm or the subcellular distribution of the viral mRNAs. Here, I describe procedures to evaluate each of these aspects of Rev/Rex function.