The polymorphic nature of double helical DNA, as expounded by Stanley Bram in the 1970s (1), is now well established (2-4). The interconversions between alternative conformations (primarily A, B, and Z) are not merely phenomena of interest to the physical biochemist but are also of central concern to the molecular biologist attempting to relate sequence features with functional attributes of consequence to gene expression and other cellular processes. It is perhaps ironic that a left-handed helical conformation was the first crystallographic structure established for DNA (5-7); it was given the deSignation of "Z-DNA" because of the peculiar zig-zag disposition of the sugar-phosphate backbone. The initial findings derived from [d(C-G)]n oligomers by Wang et al ( 5) and Drew et al (6) have been extended to numerous other sequences in which the predominant alternating purine pyrimidine motif of G' C base pairs is retained (Table 1). The Z con formation is also adopted in solution by the corresponding polymers and oligonucleotide duplexes (Table 1) in reactions accompanied by numerous changes in spectroscopic and physical attributes, as reported initially for the prototypic sequence poly[d(G-C)] (23). Z-DNA has been detected in fibers and films of DNA polyand oligomers, in supercoiled plasmids, and in chromosomes and cells.