Cited research: Science 328, 1272–1275 (2010)

A particular mutation in the H1N1 influenza virus makes it resistant to the antiviral drug Tamiflu — but it also weakens the virus, so researchers didn't think that the mutation, called H274Y, could lead to widespread drug resistance. During the 2007–08 flu season, however, resistance to Tamiflu spread rapidly.

David Baltimore and his colleagues at the California Institute of Technology in Pasadena suspected that additional viral mutations were neutralizing the weakening effects of H274Y, allowing drug resistance to flourish. The team analysed H1N1 strains gathered from 2006 onwards, and identified two such mutations.

Viruses constructed to carry these 'permissive' mutations along with H274Y grew as well as normal viruses in cells, and weren't inhibited by Tamiflu. This suggests that they enabled the sharp rise in drug resistance. A.K.