1U64:
reference: d(G3T4G4) forms unusual dimeric G-quadruplex structure with the same general fold in the presence of K+, Na+ or NH4+ ions., Sket P, Crnugelj M, Plavec J, Bioorg Med Chem. 2004 Nov 15;12(22):5735-44. PMID: 15498650

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