Although both recombinant viruses had the lower replication capacity than wild-type virus (H1N1-PR8), H1N1-PR8-NSCE1-mCherry exhibited higher replication efficiency than H1N1-PR8-NSWT-mCherry, reaching the titer of 108 TCID50/mL at 60 h post-infection (hpi), suggesting that CE1 mutation partially restored viral replication capacity compromised by mCherry gene insertion (Figure 2B). This evidence concerns the gene CES1 and infection.