XK and influenza: PB1-NA vRNA interactions were previously shown for both the A/Moscow/10/99 (H3N2) [9] and A/Finch/England/2051/91 (H5N2) [18] influenza strains, but these were relatively weak, with 15% ± 2% and 9% ± 1% of the purified RNAs forming intermolecular complexes respectively.