The eight influenza virus genome segments are associated with four viral proteins to form viral ribonucleoprotein complexes (vRNPs). The major viral protein in the RNP complex is the nucleocapsid protein (NP), which coats the RNA. The remaining proteins PB1, PB2 and PA bind to the partially complementary ends of the viral RNA. The influenza viral NP behaves like a nuclear localization sequence (NLS) containing protein. The RNP docks at the nuclear envelope only in the presence of the heterodimeric karyopherin alpha and beta complex. Once the NLS is recognized by karyopherin alpha the karyopherin beta subunit joins the complex.