A remarkable feature of the innate immune system is its ability to recognize invading microorganisms and to trigger a host defense response. Of particular interest are the recently identified Toll-like receptors (TLRs), which show homology with the Drosophila Toll protein and the interleukin-1 receptor family. The TLRs are transmembrane proteins characterized by the presence of extracellular leucine-rich repeats and an intracellular Toll/interleukin-1 receptor-like domain.