G-quadruplexes are another type of DNA secondary structure that can occur at sequences with repeating G sequences, most notably the repeating GGGTTA sequence found at the ends of human chromosomes known as G-quartets. G-quadruplex can form in the single strand DNA quite easily by forming G-quartets consisting of 4 G's base pairing to each other in a circular fashion by way of Hoogsteen base pairing. Hoogsteen base pairing involves the Watson Crick face of one G base pairing to the major groove or Hoogsteen face of another G. G-quadruplexes are formed from muliple stacks of G-quartets and can form in a multitude of ways as will be illustrated in the following tutorial with physical models.