Based on the HA phylogeny ( Figure 1A ), we further classified the reassortant viruses into three subgroups (R1, R2, and R3) with bootstrap support of 80% or better, as shown in the phylogenies containing only reassortant viruses ( Figure S4 ). Similar groupings were observed in the NA phylogeny ( Figures S2C and S4B ), although here subgroup R3 clustered with subgroup R1, and two reassortant viruses isolated in 2007 (IDN/CDC1046/07, IDN/ CDC1047/07) moved to a different subgroup. These inferred clustering patterns can be explained by multiple reassortment events, or by a single reassortment followed by divergence due to mutation and selection in different populations. We note that some group 2 viruses also cluster inside the reassortant subgroups ( Figures 1A and  S2C ) and may indicate more reassortment events; however, most of them formed polytomies close to the most recent common ancestor (MRCA) of the reassortant subgroups and had poor bootstrap support for their exact placement. As the divergence between the reassortant subgroups and other intercalating group 2 viruses are low, the three subgroups may actually be linked uninterruptedly, implying a single origin. Therefore, the times and number of reassortment events that generated the putative mosaic reassortant viruses remains elusive. We examine both the single and multiple origin hypotheses in subsequent analyses, excluding the intercalating group 2 viruses from the reassortant group.