English: Schematic of triple-stranded DNA, formed by a single-stranded DNA molecule binding to the major groove of a double-stranded (ds) DNA helix. This configuration occurs where the dsDNA is a polypurine/polypyrimidine duplex and the third strand is either polypurine or polypyrimidine, depending on its orientation. The third strand forms Hoogsteen or reversed Hoogsteen hydrogen bonds with the bases of one of the other two strands.