Influenza A viruses bear an eight-segmented single-stranded negative-sense RNA genome that is replicated in the nucleus. Newly synthesized viral RNA (vRNA) segments are exported from the nucleus and transported to the plasma membrane for packaging into progeny virions. Influenza viruses exploit many host proteins during these events, and this is the portion of the viral life cycle when genetic reassortment among influenza viruses occurs. Reassortment among influenza A viruses allows viruses to expand their host range, virulence, and pandemic potential. This review covers recent studies on the export of vRNAs from the nucleus and their transport through the cytoplasm, progressive assembly, and packaging into progeny virus particles. Understanding these events and the constraints on genetic reassortment has implications for assessment of the pandemic potential of newly emerged influenza viruses, for vaccine production, for determination of viral fitness, and for identification of novel therapeutic targets.

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    Inverted selective plane illumination microscopy video of Madin-Darby canine kidney (MDCK) cells infected with influenza A/WSN/33 PA-green fluorescent protein (GFP). MDCK cells were infected for 16 h and imaged for 30 min with an entire cell volume captured every 2 s. Scale bar = 10 μm. Video reproduced with permission from Reference 10.

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