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.