Two nomenclature conventions are used routinely in influenza research: (i) the eight segments in the influenza A genome are numbered from 1 to 8 for PB2, PB1, PA, HA, NP, NA, M and NS, respectively; (ii) There are currently 16 alleles of the HA gene termed subtypes. Likewise, there are nine alleles for NA, and two alleles for non-structural (NS) proteins. Since influenza A viruses have an unusual genomic structure, we approached the genotyping problem by first analyzing each gene segment separately. According to the above conventions and considering that the evolutionary rate varies from segment to segment, we defined a genotype as a sequential combination of the lineages for each of the eight segments in a genome. A letter was assigned to each lineage of PB2, PB1, PA, NP and M, and a number followed by a letter was assigned to each lineage of HA, NA and NS with the number representing the subtype or allele. For example, [A,D,B,3A,A,2A,B,1A] is the genotype of a human seasonal subtype H3N2 virus with PB2 lineage A, PB1 lineage D, PA lineage B, HA subtype 3, lineage A and so on, following the convention for numbering of influenza genome segments. With this nomenclature, identifying genotypes and reassortment becomes an easy task accomplished by comparing the predicted genotype against all genomes that have been classified previously.