Phylogenetic tree of H1 HA gene of influenza viruses. Horizontal distances are proportional to the minimum number of nucleotide differences required to join nodes and sequences. Numbers at the nodes indicate confidence levels in a boot strap analysis with 1,000 replications. Viruses shown in bold were used in this study and classified into 3 groups according to the animal from which they were isolated (human, avian and swine). Subtypes of viruses were H1N1 except for those in parentheses. Sw: swine, Dk: duck.