Hence, we repeated the analysis (same protocol as outlined above) for a detailed mapping of only the N1 subtype family with the difference of allowing 95% sequence identity for sequences before 2009 but keeping all new NA sequences (as of April 29 th ). A characteristic clustering emerges ( Figure 3 ) that roughly corresponds to host and geographic distributions, consistent with previous reports [25, 26] . The observed clustering is robust in respect to the method used for tree generation (same for maximum parsimony or neighbour joining trees with JTT distance and gamma-distributed variable rates). The 2009 NA is part of a cluster of avian-like swine flu H1N1 strains predominantly found in European pigs. However, previous examples of human infections from swine flu are also part of the same cluster, for example from 2005 in Thailand [27] . This indicates that, similar to the current outbreak, closely related H1N1 strains have crossed species boundaries on previous occasions as also evidenced by further reports in the literature [28] [29] [30] .