Avian influenza: Solving another piece of the H5N1 puzzle

As the H5N1 avian influenza virus continues to sweep across the globe, its potential to emerge as a human-adapted virus remains high. One crucial determinant of the species specificity of influenza viruses is the haemagglutinin (HA) protein: viruses carrying only 3 of the 16 known avian and mammalian HA subtypes (H1, H2 and H3) have adapted to humans, but in each case, these viruses have caused major pandemics.