6MC4:
reference: Crystal Structure of a Tetrameric DNA Fold-Back Quadruplex., Chu B, Zhang D, Hwang W, Paukstelis PJ, J Am Chem Soc. 2018 Nov 13. doi: 10.1021/jacs.8b10153. PMID: 30384604

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).