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. 2005 Jul 26;3(9):e300. doi: 10.1371/journal.pbio.0030300

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This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

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The maximum likelihood phylogenetic tree is rooted on a divergent set of human and swine viruses for purposes of clarity only, and all horizontal branch lengths are drawn to scale. Bootstrap values are shown for key nodes. Colors are as in Figure 1, with yellow indicating all those viruses identified as clade B viruses from the analysis of the expanded NA dataset. The phylogenetic location of the reassortment event between clades A and B was set at the position of the most basal clade A virus, defined on the basis of the NA analysis, within the older clade B. The phylogenetic locations of the two critical antigenic mutations at sites 155 and 156 are also shown.