Oligonucleotides containing runs of cytosines adopt a new kind of four-stranded structure which can be described as mutually intercalating duplexes.

roles suggested for them, but the impetus behind the recent work comes from technological developments - high resolution NMR spectroscopy and single crystal X-ray diffraction have made it possible for the first time to determine the structures in detail. Now, a new and unpredicted DNA structure has come along. This is a four-stranded structure formed by the mutual intercalation - hence its name ‘i-DNA’ - of two hemiprotonated dC.dC duplexes at acid pH. Gehring et al. [ 51 have recently reported the structure of the first ex- ample of i-DNA, formed by four strands of the oligonu- cleotide d(TQ. The structure is most clearly illustrated by the ‘pencil model’ shown in Figure