A triple-stranded nucleic acid having a first nucleic acid strand that has a region of adjacent purine nucleoside residues; a second nucleic acid strand, at least a portion of which is hydrogen bonded in a Watson-Crick manner to the region of adjacent purine nucleoside residues of the first strand; and a third nucleic acid strand, at least a portion of which is hydrogen bonded to the portion of the region of adjacent purine nucleoside residues of the first strand, the portion of the region of adjacent purine nucleoside residues to which both the second strand and the third strand are bonded defining the triple-stranded nucleic acid.