XK and infection: We theorize that the anti-avN1 immunity that resulted in improved mice survival following infection with heterologous N1-serotyped influenza viruses such as pandemic CA/07 H1N1 might have resulted from two possibilities: the phylogenetic similarity of the NA gene in avian H5N1 and swine-origin CA/07 H1N1 or the highly conserved surface exposed region in the NA structure shared by the H5N1 and CA/07 viruses [17], [18].