Plasmid DNA itself appears to have an adjuvant effect on the immune system. Bacterially derived DNA can trigger innate immune defence mechanisms, the activation of dendritic cells and the production of TH1 cytokines. This is due to recognition of certain CpG dinucleotide sequences that are immunostimulatory. CpG stimulatory (CpG-S) sequences occur twenty times more frequently in bacterially-derived DNA than in eukaryotes. This is because eukaryotes exhibit “CpG suppression” – i.e. CpG dinucleotide pairs occur much less frequently than expected. Additionally, CpG-S sequences are hypomethylated. This occurs frequently in bacterial DNA, while CpG motifs occurring in eukaryotes are methylated at the cytosine nucleotide. In contrast, nucleotide sequences that inhibit the activation of an immune response (termed CpG neutralising, or CpG-N) are over represented in eukaryotic genomes. The optimal immunostimulatory sequence is an unmethylated CpG dinucleotide flanked by two 5’ purines and two 3’ pyrimidines. Additionally, flanking regions outside this immunostimulatory hexamer must be guanine-rich to ensure binding and uptake into target cells.