2KKA:
reference: Structure of a two-G-tetrad intramolecular G-quadruplex formed by a variant human telomeric sequence in K+ solution: insights into the interconversion of human telomeric G-quadruplex structures., Zhang Z, Dai J, Veliath E, Jones RA, Yang D, Nucleic Acids Res. 2009 Nov 27. PMID: 19946019

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