Guanine-rich sequences of DNA and RNA may fold in vitro and in vivo into 
G-quadruplexes, four-stranded helical structures held together by a guanine 
core. G-quadruplexes have various biological functions, including inhibition of 
telomerase and the regulation of gene transcription and translation, and have 
become an active target for drug development, particularly for novel anticancer 
therapies. The physiological functions of G-quadruplexes are discussed in this 
review and the current knowledge of G-quadruplex ligand and drug development is 
outlined.