XK and influenza: In order to assess whether the co-selection of the Udorn PB1 and NA genes was driven by vRNA-vRNA interactions, we used an in vitro assay that previously allowed the establishment of interaction networks between the vRNAs of the human A/Moscow/10/99 (H3N2) [17] and the avian A/Finch/England/2051/91 (H5N2) [18] influenza strains.