Innate immunity to bacteria involves sensing of pathogens by pattern recognition molecules, such as Toll-like receptors. Toll proteins are mainly expressed at the plasma membrane of epithelial cells, but recently it has been proposed that the Nod molecules could represent a new group of recognition molecules that sense bacterial products within the cytoplasmic compartment. However, it has been unclear which bacterial products Nod proteins detect. In the June 6
Girardin et al. analyzed muropeptides from