Because of the specificity of toll-like receptors (and other innate immune receptors) they cannot easily be changed in the course of evolution, these receptors recognize molecules that are constantly associated with threats (i.e., pathogen or cell stress) and are highly specific to these threats (i.e., cannot be mistaken for self molecules that are normally expressed under physiological conditions).  Pathogen-associated molecules that meet this requirement are thought to be critical to the pathogens function and difficult to change through mutation; they are said to be evolutionarily conserved.  Somewhat conserved features in pathogens include bacterial cell-surface lipopolysaccharides (LPS), lipoproteins, lipopeptides, and lipoarabinomannan; proteins such as flagellin from bacterial flagella; double-stranded RNA of viruses; or the unmethylated CpG islands of bacterial and viral DNA; and also of the CpG islands found in the promoters of eukaryotic DNA; as well as certain other RNA and DNA molecules. As TLR ligands are present in most pathogens, they may also be present in pathogen-derived vaccines (e.g. MMR, influenza, polio vaccines) most commercially available vaccines have been assessed for their inherent TLR ligands capacity to activate distinct subsets of immune cells. For most of the TLRs, ligand recognition specificity has now been established by gene targeting (also known as "gene knockout"):  a technique by which individual genes may be selectively deleted in mice. See the table above for a summary of known TLR ligands.