Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Toll-like receptor 9 is a protein that in humans is encoded by the TLR9 gene. TLR9 has also been designated as CD289 (cluster of differentiation 289). The protein encoded by this gene is a member of the Toll-like receptor (TLR) family which plays a fundamental role in pathogen recognition and activation of innate immunity. TLRs are named for the high degree of conservation in structure and function seen between mammalian TLRs and the Drosophila transmembrane protein Toll. TLRs are transmembrane proteins, expressed on the cell surface and the endocytic compartment and recognize pathogen-associated molecular patterns (PAMPs) that are expressed on infectious agents and initiate signalling to induce production of cytokines necessary for the innate immunity and subsequent adaptive immunity.