XK and infection: Expression of IAV viral proteins M2, NS1, NA and PB2 relative to that of NP in different IAV mutants at 6 hr post-infection is shown in Figure 1—figure supplement 3. Detection of IAV viral proteins HA1, M1 and HA2 relative to that of NP in purified virions from different IAV mutants is shown in Figure 1—figure supplement 4. Ratio of segment 5 and segment 2 RNA sequences in purified virions is shown in Figure 1—figure supplement 5. Pairwise comparisons of the replication fitness of the mutants by competition assays is shown in Figure 1—figure supplement 6.