G-Quadruplex: A Regulator of Gene Expression and Its Chemical Targeting

G-quadruplex (G4) is an important type of nucleic acid secondary structure. An abundance of potential G4-forming sites have been shown to exist in genomes, leading to increasing interest in this research field. G4 motifs are thought to be involved in the regulation of diverse biological processes and to interact with various protein factors. Because of their important regulatory functions, G4s could have a variety of applications, the most meaningful of which is the role of G4s as potential targets of antitumor therapies. Here, we focus on the regulatory functions of G4s in tumor-related gene regulation and the use of G4s in the design of antitumor therapies, including relatively recently reported G4-related binding proteins, regulatory mechanisms, and G4-ligand designs. We also introduce G4 probes for the identification of G4 structure formation in live cells. Finally, we describe some challenges in this field and the new G4-related research field.