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In molecular biology, G-quadruplex secondary structures (G4) are formed in nucleic acids by sequences that are rich in guanine. They are helical in shape and contain guanine tetrads that can form from one, two or four strands. The unimolecular forms often occur naturally near the ends of the chromosomes, better known as the telomeric regions, and in transcriptional regulatory regions of multiple genes, both in microbes and across vertebrates including oncogenes in humans. The placement and bonding to form G-quadruplexes is not random and serve very unusual functional purposes. The quadruplex structure is further stabilized by the presence of a cation, especially potassium, which sits in a central channel between each pair of tetrads. They can be formed of DNA, RNA, LNA, and PNA, and may be intramolecular, bimolecular, or tetramolecular.