G-quadruplex is a kind of special secondary structure of nuclei acids, which 
exists in genome DNA and RNA and preferentially located in functional regions 
such as telomeres, promoters, 5' untranslated region (5' UTR) of mRNA and so on. 
G-quadruplex has been implicated by a lot of studies as an important structure 
in many biological processes including gene stability, telomere synthesis, 
transcriptional or translational regulation of gene expression and 
recombination. We here review the recent studies of G-quadruplex in genome DNA, 
RNA and G-quadruplex oligonucleotides, focusing on their biological function.