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Schematic representations of: a) a G-tetrad stabilized by a metal ion in the central cavity; b) some possible topologies for simple tetramolecular, bimolecular and unimolecular G-quadruplexes (strand polarities are indicated by arrows); c) examples of unimolecular G4s that differ in strand orientation within the G4 core; and d) types of loops that join G-rich tracts in the G4 structure.
