The Toll-like receptor (TLR) family of proteins are characterized by a highly conserved Toll homology (TH) domain, which is essential for Toll-induced signal transduction. TLR1, as well as the other TLR family members, are type I transmembrane receptors that characteristically contain an extracellular domain consisting of several leucine-rich regions along with a single cytoplasmic Toll/IL-1R-like domain. TLR2 and TLR4 are activated in response to lipopolysacchride (LPS) stimulation, which results in the activation and translocation of NFkB and sμggests that these receptors are involved in mediating inflammatory responses. TLR6 and TLR7 induce NFkB signaling upon activation, sμggesting they play a role in immune reponse. Expression of TLR receptors is highest in peripheral blood leukocytes, macrophages, and monocytes.