CpG

http://en.wikipedia.org/wiki/CpG_island
In genetics, CpG islands are genomic regions that contain a high frequency of CpG sites but to date objective definitions for CpG islands are limited. In mammalian genomes, CpG islands are typically 300-3,000 base pairs in length. They are in and near approximately 40% of promoters of mammalian genes.^[1] About 70% of human promoters have a high CpG content. Given the GC frequency however, the number of CpG dinucleotides is much lower than expected.^[2] The "p" in CpG refers to the phosphodiester bond between the cytosine and the guanine, which indicates that the C and the G are next to each other on the sequence strand be it a single or double stranded.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1345710/?tool=pmcentrez

CpG oligodeoxynucleotides (or CpG ODN) are short single stranded synthetic DNA molecules that contain a cytosine "C" followed by a guanine "G". The "p" refers to the phosphodiester backbone of DNA, however some ODN have a modified phosphorothioate (PS) backbone. When these CpG motifs are unmethlyated, they act as immunostimulants.[1] CpG motifs are considered pathogen-associated molecular patterns (PAMPs) due to their abundance in microbial genomes but their rarity in vertebrate genomes.[2] The CpG PAMP is recognized by the pattern recognition receptor (PRR) Toll-Like Receptor 9 (TLR9), which is only constitutively expressed in B cells and plasmacytoid dendritic cells (pDCs) in humans and other higher primates.[3]

http://en.wikipedia.org/wiki/CpG_Oligodeoxynucleotide