Toll, a Drosophila receptor molecule with extracellular leucine-rich repeat (LRR), has a role in triggering innate defenses against bacteria or fungi. Toll-like receptor 4 (TLR4/CD284) is a member of TLR family, which is human homologue of Toll protein. It has extracellular LRR and an intracellular signaling domain, which is similar to the type I IL-1 receptor. TLR4 is expressed in subpopulations of cells including myeloid cells, B cells, monocytes, and endothelial cells. Recent studies have suggested that TLR4 might act as a receptor for lipopolysaccharide (LPS). TLR4 alone is not capable of sensing and signaling the presence of LPS, but another molecule MD-2, which is physically associated with TLR4, is required for LPS recognition through TLR4.