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 telomerase activity, gene regulation, and functional genomics. Guanine analogues that have different hydrogen bonding characteristics - 7-deaza-8-aza-dG and 7-deaza-dG - have proved useful in analyzing G-quadruplex structures. Similarly, common DNA lesions - 8-oxo-dG and abasic sites - have been used to investigate their effect on G-quadruplex structure and activity.