Oligonucleotide structural analysis has demonstrated that DNA and RNA nucleic acid sequences containing G-tracts separated by other bases spontaneously fold into G-quadruplex structures. G-quadruplexes are formed when four adjacent guanine residues stack in a cyclic Hoogsteen hydrogen-bonding arrangement leading to four-stranded helical structures. The study of G-quadruplexes in basic genetic processes is an active area of research in telomere, gene regulation, and functional genomics research.