The Toll signalling pathway, which is required for establishment of dorsoventral polarity in Drosophila embryos, plays an important role in the response to microbial infections. Recently, Toll-like receptors (TLRs) have also been identified in mammals. TLR4 has been shown to function as the transmembrane component of the lipopolysaccharide receptor, while TLR2 recognizes peptidoglycans from Gram-positive bacteria, lipoproteins and yeast. Although various microbial cell-wall components are recognized by different receptors, all of these responses are abrogated in MyD88-deficient cells. These results show that different TLRs recognize different microbial cell-wall components, and that MyD88 is an essential signalling molecule shared among interleukin-1 receptor/Toll family members.