Historical Perspective. Theformation of a specific complex between two strands of poly (uridylic acid) with one strand of poly (adenylic acid) in thepresence of divalentcations was first described by Felsenfeld et al.(1957). Subsequently, triple-stranded complexes containing other combinations of polynucleotide strands were described including those belonging to the deoxyribose series [reviewed in Felsenfeld and Miles (1967), Wells et al.(1988), Cheng and Pettitt (1992b), and Sun and H61Sne (1993)]. These studies culminated in lowresolution, X-ray fiber-diffraction models for the (A)„-2 (U)„,(A)„’2 (I)„, and (A)„-2 (T)„combinations (Arnott & Seising, 1974, and references cited therein). Further, a potential biological function for these structures was identified when they were found to inhibit RNA1polymerase-mediated transcription in vitro (Morgan & Wells, 1968). Interest in triplexes revived following the discovery of single-strand-specific SI endonuclease hypersensitive sites in the upstream regions of several eukaryotic genes (Larsen & Weintraub, 1982). These sites were mapped to (R)„-(Y)„sequences which, when subcloned into supercoiled plasmids, exhibited the same sensitivity toward SI nuclease [reviewed