Expansion of (AGC)n repeats has been associated with genetic disorders called triplet-repeat diseases such as Huntington's disease (HD), myotonic muscular dystrophy (DM) and Kennedy's disease. To gain insight into the abnormal behavior of these repeats, we studied their structural properties in supercoiled DNA. Chemical probing revealed that, under physiological salt and pH conditions, Zn2+ or Co2+ ions induce (AGC)n repeats to adopt a novel non-B DNA structure in which all cytosine but none of adenine residues in either strand become unpaired. The minimum size of (AGC)n repeat that could form this structure independently of neighboring sequences is a single unit of double-stranded trinucleotide, 5'AGC3'/5'GCT3'. Other trinucleotide units of the same nucleotide composition, 5'CAG3'/5'CTG3' or 5'GCA3'/5'TGC3', do not form non-B DNA structures. This unusual DNA structural properly adopted by a single 5'AGC3'/5'GCT3' trinucleotide may contribute to expansion of (AGC)n sequences in triplet-repeat diseases.