Toll signal transduction pathway plays a critical role in development as well as in innate immune responses in Drosophila. Positional cloning work proceeded in mice recently has revealed that Lps encodes the Toll-like receptor 4 (TLR4) of mammals, which functions as the transmembrane component of LPS receptor complex. In contrast, TLR2, a receptor closely related to TLR4, makes no contribution to LPS induced signaling. The discovery of TLR4 has greatly promoted understanding on the LPS signal transduction pathway.