In this study, we report the first atomic resolution structure of a stable G-hairpin formed by a natively occurring DNA sequence. An 11-nt long G-rich DNA oligonucleotide, 5'-d(GTGTGGGTGTG)-3', corresponding to the most abundant sequence motif in irregular telomeric DNA from Saccharomyces cerevisiae (yeast), is demonstrated to adopt a novel type of mixed parallel/ antiparallel fold-back DNA structure, which is stabilized by dynamic G:G base pairs that transit between NI -carbonyl symmetric and N1-carbonyl, N7-aminobase-pairingarrangements. Although the studied sequence first appears to possess a low capacity for base pairing, it forms a thermodynamically stable structure with a rather complex topology that includes a chain reversal arrangement of the backbone in the center of the continuous G-tract and 3' to -5' stacking of the terminal residues. The structure reveals previously unknown principles of the folding of G rich oligonucleotides that could be applied to the prediction of natural and/or the design of artificial recognition DNA elements. The structure also demonstrates that the folding landscapes of short DNA single strands is much more complex than previously assumed.