Four inosine bases (Is) can be cyclically and planarly associated through four hydrogen bonds to form a macrocycle called an I-quartet, in a manner similar to that of the well-known guanine (G)-one. We identified and characterized 1-quartets formed in parallel DNA quadruplexes using H-1 NMR. Comparative studies on I- and G-quartets formed in parallel DNA quadruplexes revealed remarkable differences in their physicochemical properties that could be interpreted in terms of buckling of the quartet plane.