Mammals possess several lines of defense against microbes. One of them is activated when receptors called Fprs, which are present on immune cells, bind to specific molecules that are linked to pathogens. Researchers at the University of Geneva (UNIGE), Switzerland, showed in 2009 that these same receptors were also present in the nose of mice, probably to detect contaminated food or to avoid sick conspecifics. The biologists now describe in the journal PNAS how Fprs have acquired this olfactory role during rodent evolution, moving from the immune system to a neuronal system. This innovation results from two genomic ‘accidents’ that occurred several millions years apart during the evolution of rodents.