At three days post-infection (p.i.), titres for PB1-NA-PB1, HA-NA-HA, NP-NA-NP, M-NA-M and NS-NA-NS were reduced by 2–4 log10, and those for PB2-NA-PB2 and PA-NA-PA were reduced by 1–2 log10 compared to wild-type (NA-NA-NA) (Fig. 1c). This evidence concerns the gene XK and infection.