English: (A) A putative quadruplex sequence (PQS) is a nucleotide sequence predicted to form a G4 structure. A degenerate PQS used to predict the formation of intramolecular G4s is shown here, consisting of four runs of at least three guanines per run, separated by short stretches of other bases (N). (B) The basic unit of the G4 is the G-tetrad. (C) G4 structures display a large variety of different topologies. Topology of intramolecular G4 structures displaying antiparallel (left) and parallel (right) configurations. (D) Topology of intermolecular G4 structures formed by dimerisation of four strands (left) or two strands (right). Harris LM, Merrick CJ (2015) G-Quadruplexes in Pathogens: A Common Route to Virulence Control? PLoS Pathog 11(2): e1004562. doi:10.1371/journal.ppat.1004562