Single‐Cell Visualization of Monogenic RNA G‐quadruplex and Occupied G‐quadruplex Ratio through a Module‐Assembled Multifunctional Probes Assay (MAMPA)

G-quadruplexes (G4s), non-canonical nucleic acid secondary structure, regulate many biological functions and are considered potential molecular targets for cancer therapeutics. However, due to the lack of analytical methods, the regulating mechanism of monogenic G4s is still unclear. Here, we developed a Module Assembled Multifunctional Probes Assay (MAMPA) for visualizing endogenous G4s in individual genes in single cells. Two modular probes separately recognize G4 structures and the adjacent RNA sequences, and the module assembly enables imaging of G4s in an individual RNA with high specificity. Through imaging G4s in several individual genes, we found that G4s were steadily occupied by G4 Binding Proteins (G4BPs) in various mRNAs in every cell line and defined "Occupied G4 Ratio". We demonstrated MAMPA was suitable for most experimental situations and found that Occupied G4 Ratios had the potential to become a new parameter for the study of G4s in living cells.