Multidimensional heteronuclear NMR study has demonstrated that a guanine-rich DNA oligonucleotide originating from N-myc gene folds into G-quadruplex structures in the presence of K+, NH4+ and Na+ ions. A monomeric G-quadruplex formed in K+ ion containing solution exhibits three G- quartets and flexible propeller-type loops. The 3D structure with three single nucleotide loops represents a missing element in structures of parallel G-quadruplexes. The structural features together with the high temperature stability are suggestive of specific biological role of G-quadruplex formation within intron of N-myc gene. Increase in K+ ion and oligonucleotide concentrations resulted in transformation of monomeric G-quadruplex into a dimeric form. The dimeric G-quadruplex exhibits six stacked G- quartets, parallel strand orientations and propeller-type loops. A link between the third and the fourth G- quartets consists of two adenine residues that are flipped out to facilitate consecutive stacking of six G- quartets.