DNA exists in many possible conformations that include the A DNA, B DNA, and Z DNA forms; of these, B DNA is the most common form found in cells. The DNAs that do not fall into a right handed Watson Crick double helix are known as non B DNAs and comprise cruciform, triplex, slipped (hairpin) structures, tetraplex (G quadruplex), left handed Z DNA, and others. Several recent publications have provided significant evidence that non B DNA structures may play a role in DNA instability and mutagenesis, leading to both DNA rearrangements and increased mutational rates, which are hallmark of cancer.