DNA triple helix (triplex) structures. A) Three-dimensional view of an intramolecular DNA triplex solved by solution-state nuclear magnetic resonance (PDB ID: 1BCB). The structure was deposited by Asensio et al. (1998) and classified as DNA without mutations.⁽²⁰⁾ This triple helical arrangement illustrates the association of a third strand with the canonical duplex, stabilized by Hoogsteen interactions. B) Simplified schematic (adapted from Brazda et al., 2020; and Holder et al., 2015) illustrating the principle of triplex formation.^(22,23) The canonical duplex is stabilized by Watson–Crick base pairs, while the third strand binds in the major groove via Hoogsteen hydrogen bonds. The chemical structure shown in green corresponds to adenine from the third strand, forming a representative T·A·T triad with a Watson–Crick A·T base pair. Such alternative base-pairing interactions enable an additional strand to associate with duplex DNA through Hoogsteen hydrogen bonding, generating a triple-helical structure capable of modulating essential processes such as replication and transcription