"Nucleic acids are intrinsically polymorphic. Apart from the Watson-crick duplex, guanine-rich sequences can form G-quadruplex (G4) structures, which have become attractive targets for small molecule ligands. It is now proven that G-quadruplexes can form in cells. Today most biophysical and structural/ligand binding studies are carried out in dilute aqueous solutions and the presence of artificial crowding agents/cosolvents. Still, recent works suggest that the folding of G4s may differ in the cellular milieu context due to the agents' limitations. Thus there is an immense need for in-cell-based structural biology approaches. In the proposal, I aim to address critical milestones leading to the bioactive conformation inside the cell and the development of effective/specific G4 targeting drugs using an essential sub-class of G4 structures as a paradigm, namely quadruplex−duplex hybrids (QDH-hereafter). We will focus on low- and high-resolution spectroscopic approaches and mass spectrometry-based ligand screening assays characterizing ligand-QDH interaction. On the methodological part, we will develop in-cell NMR in ""native"" conditions (starting from unfolded conformation: denovo folding) and propose a ligand screening assay inside the cellular system. Our project will contribute to unveil fundamental principles of nucleic acid folding. It will also foster collaboration between two European institutes specialized in complementary biophysical and structural approaches to study biomolecular folding. In a nutshell, the Maria Skłodowska Curie fellowship will flourish my scientific excellence and the resilience needed for my original ideas to become a reality and communication and public engagement skills across cultures and disciplines."