G-quadruplexes form folded structures because of tandem repeats of guanine 
sequences in DNA or RNA. They adopt a variety of conformations, depending on 
many factors, including the type of loops and cations, the nucleotide strand 
number, and the main strand polarity of the G-quadruplex. Meanwhile, the 
different conformations of G-quadruplexes have certain influences on their 
biological functions, such as the inhibition of transcription, translation, and 
DNA replication. In addition, G-quadruplex binding proteins also affect the 
structure and function of G-quadruplexes. Some chemically synthesized 
G-quadruplex sequences have been shown to have biological activities. For 
example, bimolecular G-quadruplexes of AS1411 act as targets of exogenous drugs 
that inhibit the proliferation of malignant tumours. G-quadruplexes are also 
used as vehicles to deliver nanoparticles. Thus, it is important to identify the 
factors that influence G-quadruplex structures and maintain the stability of 
G-quadruplexes. Herein, we mainly discuss the factors influencing G-quadruplexes 
and the synthetic G-quadruplex, AS1411. SIGNIFICANCE OF THE STUDY: This review 
summarizes the factors that influence G-quadruplexes and the functions of the 
synthetic G-quadruplex, AS1411. It also discusses the use of G-quadruplexes for 
drug delivery in tumour therapy.