You have modified the green fluorescent protein (GFP) to carry a nuclear localization signal at its N-terminus and a nuclear export signal at its C-terminus. When this protein is expressed in cells, you find that about 60% of it is located in the nucleus and the rest in the cytoplasm and that its rates of import and export determine the observed distribution. Nevertheless, you are concerned that the modified GFP might not fold properly even though it retains its fluorescent properties. Specifically, you worry that the 40% in the cytoplasm is not being transported at all because neither signal is exposed, and the 60% in the nucleus has only the nuclear import signal available. To test this, you may treat cells with leptomycin B, a drug that inhibits nuclear export. How might addition of leptomycin B to such cells resolve this issue