Illustration of a dimeric interlocked parallel DNA (deoxyribonucleic acid) G-quadruplex molecule formed from synthetic DNA. DNA G-quadruplexes form in sequences that are rich in the nucleotide guanine (G). The structure is further stabilised by the presence of a cation, in this case potassium ions (red spheres). They are found in regions at the end of chromosomes called telomeres and in areas of genes that regulate transcription.