While antigenic drift of HA is clearly of vital importance in the survival of an influenza strain, other factors, including HA receptor binding specificity [19], antigenic drift of neuraminidase (NA) [20], matched activity between HA and NA [21–23], and the interaction of the other influenza proteins with each other and their host cells, are all likely to affect viral fitness in a polygenic manner. This evidence concerns the gene XK and influenza.