Guanine- and cytosine-rich sequences may fold into tetrahelical structures called G-quadruplexes and i-motifs undercertain conditions, respectively. G-quadruplexes are noncanonical fourstrandedstructures consisting of stacks of guanine residuesassembled into G-quartets and coordinated with intercalated cations such as potassium and sodium. These structures exist in dynamic equilibriumwithin the single-stranded G-rich DNA generatedduring major genomic events (replication, transcription).G-quadruplexes may be modulators of nucleic-acid-processingproteins and, as such, as potential components of newpathways of genome and epigenome regulation.Solution-state NMR spectroscopy has contributedsignificantinsights that helped to uncover overall topologies and local features of non-B-DNA structural families alone or in interaction with other molecules such as small molecule ligands. An unexpected four-stranded structures stabilized by G-A and G-C base pairs stimulated us to explore if G- and A-rich repeat segments of DNA can adopt tetrahelical structures different from G-quadruplexes. 5'-AGCGA-3' repeat sequences are found in regulatory regions of 38 different human genes linkedto neurodevelopment and neurological disorders, abnormal cartilage and bone formations, cancer and regulation of basic cellular processes. In contrast to the expected G-quartet-based topologies adopted by 5'-GGG-3' repeats structures are stabilized by G-C, G-A and G-G base pairs that interact to form unique structures. In comparison to G-quadruplexes novel structural family does not show the same sensitivity to the presence of cations. New structures suggest that folding landscapes and structural diversity of DNA oligonucleotides are much more complex than previously assumed.