The derivation of H5N1 reassortants in the years 2000 through 2003. Reassortment between influenza viruses is proposed to generate viruses with different gene constellations. Gs/Gd/96-like viruses reassorted with viruses from wild aquatic birds and multiple H5N1 genotypes appeared in Hong Kong poultry markets in 2001. Some 2001 H5N1 genotypes may have been transmitted back from domestic poultry to the wild aquatic avian reservoir, where the next reassortant events may have occurred. As a result, multiple H5N1 genotypes, mostly different from those of 2001, were isolated from domestic poultry and wild birds in Hong Kong in 2002-2003. The eight gene segments schematically shown in each virus particle encode (top to bottom) polymerase complex (PB2, PB1, and PA), HA, nucleoprotein (NP), NA, matrix (M), and nonstructural (NS) proteins. Color coding indicates virus lineages. A gap in the NA or NS gene segment denotes a deletion. Deletions within the NA gene appear to result from adaptation to poultry, although the exact role of this phenomenon is unclear. NS gene deletions in H5N1 viruses have recently been observed, but the biological significance of these deletions is unknown.