1XCE:
reference: Experimental Demonstration of T:(G:G:G:G):T Hexad and T:A:A:T Tetrad Alignments within a DNA Quadruplex Stem., Webba da Silva M, Biochemistry 2005 Mar 15;44(10):3754-64. PMID: 15751952

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