TLR4 (Toll-like receptor 4) is a mammalain homologue of the Drosophila toll protein. The human TLR4 is a type-1 transmembrane protein containing 799 amino acid residues. The extracellular domain has leucine-rich repeats and the intracellular domain is similar to that of IL-1 receptor. TLR4 is a receptor for lipopolysaccharide (LPS). In mice that are genetically tolerant to endotoxin, mutations have been identified in the intracellular domain of TLR4 that abolish LPS responsiveness. TLR4 uses a mechanism similar to that of the IL-1 receptor for signal transduction that leads to activation of NF-kB.