Genomic sequences of all influenza A viruses with 475% of the full segment length were downloaded from NCBI Influenza Virus Resource (http://www.ncbi.nlm.nih.gov/ genomes/FLU/FLU.html). Alignments were performed for each individual gene segment using the ClustalW program (22) . The MEGA software was used to construct the phylogenetic trees with the neighbor-joining method and the HKY-85 model selected (23) . The goal of our genotype method is to determine when a reassortment event with a gene segment from a non-traditional host or location has occurred. The lineages of each viral gene were carefully determined as detailed subsequently: (i) using the phylogenetic trees constructed, significant clusters (which were segregated by $10% nucleotide difference by p-distance) were assigned lineages; (ii) bootstrap analysis was used on a smaller set of sequences with values 490% considered significant; (iii) the initial lineages were evaluated for nucleotide differences within and between other lineages and for strength of bootstrap support; (iv) approximately 10 sequences from each lineage were randomly selected for the maximum likelihood (ML) analysis for each gene segment, serotype (for HA, NA) or allele (for NS) on the MultiPhyl server (24) . The lineage assignment of each influenza gene available in the public databases was uploaded into the Segment Table in the database as described subsequently.