ABSTRACT
<p><b>OBJECTIVE</b>To study the global evolutionary characteristics of hemagglutinin gene HA1 of influenza H1N1 infecting different species during 2000-2009.</p><p><b>METHODS</b>The target sequences were downloaded from NCBI and analyzed using bioinformatic software to construct the phylogenetic tree.</p><p><b>RESULTS</b>The HA1 amino acid sequences of influenza H1N1 contained four mutated antigenic sites and receptor-binding sites, and the novel influenza virus shared most of the mutated amino acid sites with swine H1N1 influenza virus.</p><p><b>CONCLUSION</b>The HA1 gene of novel influenza virus might originate from the early swine H1N1 influenza virus from North America, and in the evolutionary process, a number of important sites of HA1 gene mutated to result in the outbreak of influenza.</p>