The influenza A genome consists of eight negativestranded RNA segments that encode at least 10 viral proteins (12) . The viral genome evolves through accumulation of mutation by the viral RNA-dependent RNA polymerase which lacks proofreading ability and through reassortment of entire gene segments (13) . Forces selecting viral variants such as the neutralizing antibody response of vertebrate hosts as well as species-related structural variation can also promote rapid evolution (14) . Each of the segments can evolve at a different rate if they are subject to differential selective pressures and functional constraints (15) (16) (17) (18) (19) . The segmented nature of the viral genome allows for segment exchange (termed reassortment) when two distinct viruses co-infect a cell and generate progeny with a mixed genome (20, 21) . Reassortment may theoretically yield 254 (2 8 -2) different combinations of gene segments from two parent viruses.