We further annotated the tree nodes with their most probable In combination with other topological differences between the trees, this difference strongly suggests past reassortment events between both segments, with the progenitor virus of the basal Hong Kong clade and a chicken strain from Hebei having acquired an NA segment from different lineages. Such events are not surprising given frequent reports of A-H5N1 reassortment in China, e.g. [26] , and the particular reassortment event for the basal Hong Kong clade has very recently been confirmed [32] .