A controversial study of the H5N1 bird flu virus has been published today in the journal Nature. The study, and similar one that will be published in Science, sparked an international furor when it was accepted for publication, sparking concerns that the work, which included mutating H5N1 to be transmissible among mammals, could inspire bioterrorism. In the wild, bird flu is deadly, but does not spread easily between humans. In the published study, Yoshihiro Kawaoka and his colleagues at the University of Wisconsin–Madison, mutated the virus’ haemagglutinin (HA) gene, which produces a protein the virus uses to attach to host cells. The researchers found that the virus was able to spread among ferrets in different cages after just four mutations. While this mutated version of the virus was not as deadly, it provided valuable insight into the mechanisms and implications of mutation. The researchers hope their will help health agencies recognize key factors that could predict a pandemic.