Base composition signatures provide a multidimensional fingerprint of the genomes of the various viruses, which can be used to determine clusters of related species/sub-types. One such representation ( Figure 2 ) shows base composition data derived from the PA, PB1, and NP gene segments analyzed on individual axes. Importantly, only three of the six influenza A primer pairs from Figure 1 are visualized on this three-dimensional plot. There was strong agreement between the bioinformatic analysis of sequence data from GenBank and experimental measurements of base composition signatures from Figure 1 . Human H3N2 and H1N1 viruses clustered independently from each other and from the avian/human H5N1 and H1N1 viruses.