As part of the innate immune response, Toll-like receptor 9 recognizes unmethylated CpG nucleotide sequences, which are very common in viruses and prokaryotes and more rare in vetebrates. Scientists routinely use synthetic oligonucleotides as TLR9 agonists for a variety of applications requiring activation of an innate immune response via TLR9. These relatively simple oligonucleotides are easy to produce, and as a result some companies have extensive catalog offers.

TLR9 structure in complex with CpG ODN. Source: Zhou et al. doi:10.1186/1742-4682-10-18.