A family of unusual DNA structures has been discovered in segments with predominantly purines in one strand (pur.pyr sequences). These sequences are overrepresented in eukaryotic DNA and have been mapped near genes and recombination hot spots. When cloned into recombinant plasmids, many pur.pyr sequences are reactive to chemical and enzymic probes that are generally specific for single-stranded DNA. An intramolecular triplex is adopted by mirror repeats of G's and A's. Other non-B DNA structures adopted by similar sequences remain to be fully clarified but may be a family of related conformations. It is likely that these unorthodox structures play an important role in the function of the eukaryotic genome.

A veritable explosion in our knowledge of unusual DNA structures has occurred in recent years. Our enhanced knowledge about specific DNA sequences via DNA sequencing and cloning has been a contributory factor. Furthermore, the establishment of diverse types of methods for the detection and analysis of unusual DNA structures has also been important. For example, in the early days of left-handed Z-DNA (1979–1982), the only techniques available were x-ray diffraction on short oligonucleotides, circular dichroism, phosphorus NMR, and laser Raman spectroscopy. Today, at least 15 different types of physical, enzymatic, immunological, chemical, and spectroscopic methods are available. Studies have been conducted on recombinant plasraids, short synthetic oligonucleotides, as well as purified restriction fragments.