2.5 Virus rescue.
The eight gene segments of GX/18 and GD/104 were inserted into the vRNA–mRNA bidirectional transcription vector pBD to rescue rGX/18, rGD/104 and the reassortant and mutant viruses as described previously(Ping et al., 2008). Mutations were introduced into HA gene in the background of GX/18 and GD/104 by site-directed mutagenesis (Invitrogen) according to the manufacturer’s protocol. Gene-specific primers used in study were shown in Table 1, and all the rescued viruses were completely sequenced to avoid the unwanted mutations.