G-quadruplexes (G-tetrads or G4-DNA) are nucleic acid sequences that are rich in guanine and are capable of forming a four-stranded structure. Four guanine bases can associate through Hoogsteen hydrogen bonding to form a square planar structure called a guanine tetrad, and two or more guanine tetrads can stack on top of each other to form a G-quadruplex. 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. Depending on the direction of the strands or parts of a strand that form the tetrads, structures may be described as parallel or antiparallel.