The influenza virus non-structural protein NS2, which had no previously assigned function, was shown to mediate the nuclear export of virion RNAs by acting as an adaptor between viral ribonucleoprotein complexes and the nuclear export machinery of the cell. NS2, also called nuclear export protein (NEP), of influenza A virus contains a leucine-rich nuclear-export signal that could guide viral ribonucleoproteins to cross the nuclear pore complex (NPC) and complete directional nucleocytoplasmic trafficking. The NS2 (NEP) protein contains a highly conserved nuclear export signal (NES) motif in its amino-terminal region (12ILMRMSKMQL21, A/WSN/33), which is thought to be required for nuclear export of viral ribonucleoprotein complexes (vRNPs) mediated by a cellular export factor, CRM1. The NS2 is a phosphoprotein and binds to the nucleoporin yRip1 and Rab/hRip as well as the M1 protein which associates with the vRNPs. Therefore, the NS2 protein plays an important role in the nuclear export of the vRNPs.