Phylogeny estimated for the NA protein-coding gene. Sequences from the 2009 H1N1 pandemics and the clade sustaining them are in red. Host switch events are marked with a star (*): in black for swine-to-human event and in gray for human-to-swine events. Branch lengths are proportional to time; the horizontal axis below the tree gives the scale.
Abdussamad and Aris-Brosou BMC Evolutionary Biology 2011 11:6 doi:10.1186/1471-2148-11-6