XK and influenza: To validate the use of R3∆PB1 viruses for influenza neutralization assays, we also rescued replication-competent H1N1 and H3N2 parental viruses, which possess HA and NA of wild-type viruses and the internal genes including PB1 of A/WSN/1933, and propagated in MDCK-SIAT1 cells (Fig. 1b, c).