Mutations in the polymerase gene could explain how the influenza virus jumps the species barrier, according to a study in this week's Proceedings of the National Academy of Sciences. The mutations appear to enhance the enzyme's activity, making the virus more virulent. Similar mutations are also present in the highly pathogenic avian flu virus H5N1, strains of which have recently been detected in mammals. These findings suggest that changes to polymerase may be a prerequisite for adaptation to a new host, opening the door to a potential pandemic, according to study author Juergen Stech, who headed the team from the Institute of Virology in Marburg, Germany.