Phylogenetic trees for the NA gene of A(H1N1) viruses. The region from nucleotide 21 to 1430 (1410 nucleotides) for the NA gene were used for the analysis. The numbers below the branches are the bootstrap probabilities (percentages), showing only values greater than 70%. Viruses with a H275Y mutation in the NA gene isolated in Yamagata are shown in red, and those isolated in Europe, the USA, South Africa, and Japan (except Yamagata) are shown in green. Viruses with H275 strains in Yamagata are shown in blue. Viruses from the 2008-2009 season are indicated in boldface.