To examine whether the residues at H9N2 NA positions 198,
199 and 338 have undergone mutations, we examined 1130 fulllength
N2 sequences of H9N2 virus available in GenBank (as of
February 16, 2015) using MUSCLE. As shown in Table 2, position
198 is comparatively conservative, as evidenced by only 0.1% of the
sequences analyzed, however positions 199 and 338 are highly
variable, with various mutations having naturally occurred.
Interestingly, the D198N and K199E mutations detected in the NA
of escape mutants obtained in the present study are already present
in some of the natural H9N2 isolates. These findings suggest that
antibody selection pressure on the antigenic area of H9N2 virus NA
in the field.