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The Toll-like receptor (TLR) family in mammal comprises a family of transmembrane proteins characterized by multiple copies of leucine rich repeats in the extracellular domain and IL-1 receptor motif in the cytoplasmic domain. Like its counterparts in Drosophila, TLRs signal through adaptor molecules. Toll-like receptor 4 (TLR4), the principal signaling receptor for lipopolysaccharide (LPS) in mammals, requires the binding of MD-2 to its extracellular domain for maximal responsiveness. MD-2 enables TLR4 to respond to a wide variety of LPS partial structures, Gram-negative bacteria, and Gram-positive lipoteichoic acid, but not to Gram-positive bacteria, peptidoglycan, and lipopeptide.