A structure-specific antibody, BG4, was developed and used to visualize DNA G-quadruplex structures in human cells. The antibody shows high affinity for G-quadruplexes and no binding to other nucleic acid structures. The study demonstrates that G-quadruplex formation in DNA is modulated during the cell cycle, with increased formation during S phase, when DNA replication occurs. Additionally, a small molecule ligand, pyridostatin (PDS), stabilizes G-quadruplex structures in cells, increasing the number of BG4 targets. The antibody was used to visualize G-quadruplex structures in human cells, showing their presence at chromosomal ends (telomeres) and in other regions of the genome. The results indicate that G-quadruplex structures are present in mammalian cells and can be targeted by small molecule ligands. The findings support the role of G-quadruplex structures in genome function and highlight their potential as targets for therapeutic intervention. The study provides a method for quantitatively visualizing G-quadruplex structures in human cells, which could aid in understanding their biological roles and developing targeted therapies.A structure-specific antibody, BG4, was developed and used to visualize DNA G-quadruplex structures in human cells. The antibody shows high affinity for G-quadruplexes and no binding to other nucleic acid structures. The study demonstrates that G-quadruplex formation in DNA is modulated during the cell cycle, with increased formation during S phase, when DNA replication occurs. Additionally, a small molecule ligand, pyridostatin (PDS), stabilizes G-quadruplex structures in cells, increasing the number of BG4 targets. The antibody was used to visualize G-quadruplex structures in human cells, showing their presence at chromosomal ends (telomeres) and in other regions of the genome. The results indicate that G-quadruplex structures are present in mammalian cells and can be targeted by small molecule ligands. The findings support the role of G-quadruplex structures in genome function and highlight their potential as targets for therapeutic intervention. The study provides a method for quantitatively visualizing G-quadruplex structures in human cells, which could aid in understanding their biological roles and developing targeted therapies.