FIGURE 2-3 The genotypes of H5N1 influenza virus reassortants from eastern Asia. The eight gene segments are (horizontal bars starting at the top downwards): PB2, PB1, PA, HA, NP, NA, M, and NS. Each colour represents a virus lineage (red indicates origin from Gs/Gd/1/96). Genotypes (indicated by letters) were defined by gene phylogeny: a distinct phylogenetic lineage with bootstrap support ≥70% (≥50% for M, NP, and PA genes) indicated a common origin. Genotypes A, B, and C were reassortants of Gs/Gd/1/96 and one or more aquatic avian viruses. Genotype D was created when the NP gene of genotype C was replaced by that of a Dk/HK/Y280/97-like virus (H9N2 subtype). Genotype E was created when the NP gene of genotype C was replaced by that of another avian virus. Further reassortment of genotype E with other aquatic avian influenza viruses gave rise to the genotypes X₀−X₃, distinguished by the sources of their PB2, PA, and NS genes. Genotype W differs from genotype B only in its PB2, NP, and M genes. Further reassortment of genotype A or B with other aquatic avian viruses gave rise to genotypes V, Y, Z, and Z⁺. Alternatively, genotype V may have resulted from reassortment of genotype Z with other aquatic avian viruses.