Certain nucleic acids sequences can form unusual structures, like triplexes, G-quadruplexes and i-motif. Nowadays, several genetic diseases have proved to be mediated by the formation of non-B DNA structures at certain chromosomal locations. Therefore, it is very important to inversigate these unusual DNA structures, which may well be biologically relevant, at the molecular level. To study the function of such unusual DNA structures in vivo, it is important to understand how they assemble and interact with ligands. Until now, most of the these studies are carried out in dilute solutions, which do not reflect the real conditions in vivo because the physiological milieu is crowded with various biomolecules. Moreover, current research methods related to unusual DNA structures are ensemble methods, a lot of important phenomena may inherent in ensemble measurements due to population averaging.Magnetic tweezers are extraordinarily sensitive, as this technique is capable of applying piconewton-scale forces, allowing it to provide direct data on the molecular-scale workings of biological systems. Therefore, the magnetic tweezers technique is well suited to experiments on single DNA molecules.The proposal we submitted is the first attempt to investigate unusual DNA structures by magnetic tweezers. We aim to understand how they behave under near-physiological conditions and how the interaction with ligands may affect their behavior. The results can promote other researchers to learn the formation pathways of these unusual DNA structures, and help them to effectively design anticancer drugs or to understand how these structures may interfere with key biological processes. To our knowledge, such study of unusual structures has never been performed – or at least published – with this method. Through detailed measurements of the force-extension relationship of unusual nucleic acids, opened up a new way of investigating these atypical structures.