| Summary: | Identification of the existence of G-quadruplex (G4) structure, from a specific G-rich sequence in cells, is critical to the studies of structural biology and drug development. Accumulating evidence supports the existence of G4 structure in vivo. Particularly, time-gated fluorescence lifetime imaging microscopy (FLIM) of a G4 fluorescent probe, 3,6-bis(1-methyl-2-vinylpyridinium) carbazole diiodide (<i>o</i>-BMVC), was used to quantitatively measure the number of G4 foci, not only in different cell lines, but also in tissue biopsy. Here, circular dichroism spectra and polyacrylamide gel electrophoresis assays show that the use of antisense oligonucleotides unfolds their G4 structures in different percentages. Using antisense oligonucleotides, quantitative measurement of the number of <i>o</i>-BMVC foci in time-gated FLIM images provides a method for identifying which G4 motifs form G4 structures in fixed cells. Here, the decrease of the <i>o</i>-BMVC foci number, upon the pretreatment of antisense sequences, (CCCTAA)<sub>3</sub>CCCTA, in fixed cells and at the end of metaphase chromosomes, allows us to identify the formation of telomeric G4 structures from TTAGGG repeats in fixed cells.
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