Influenza A viruses are classified into subtypes on the basis of antigenic analysis of hemagglutinin (HA) and neuraminidase (NA) glycoproteins. So far, 16 HA subtypes and 9 NA subtypes have been found (10) . In recent years, gene sequences have become available for a large number of viral strains creating a diverse pool of influenza A viruses from historical and current isolates collected in multiple geographic regions. Comparison of the deduced amino acid sequences of the HA and NA revealed an excellent agreement between the results of clustering viruses by the antigenic reactivity and sequence similarity. However, molecular genetic analysis allows a comprehensive analysis of the entire viral genome and is gaining popularity because it is more practical for most laboratories as a method for classification (11) . Most importantly, study of the influenza genomic structure, namely genotyping, could reveal mechanisms of virus evolution, spread and disease pathogenesis.