Over the past decade, many papers have examined the binding of ethidium and ethidium analogs to quadruplex DNA.  The results from these manuscripts indicate considerable disagreement in whether ethidium binds to quadruplex DNA.  Our laboratory investigated the binding of ethidium to the Na⁺ form of G-quadruplex DNA using both spectroscopic and calorimetric techniques.  Our data indicates ethidium does indeed bind G-quadruplex DNA as shown by isothermal titration calorimetry (ITC).  The binding is characterized by a favorable binding enthalpy (ΔH_obs of -8 kcal/mol) to the human telomere quadruplex DNA.  Circular dichroism spectroscopy is commonly used to characterize structural aspects of macromolecules.  The titration of ethidium into G-quadruplex DNA revealed no shifts in the CD spectrum, resulting in a spectrum with a maximum at 298 nm, minimum at 265 nm, and maximum at 245 nm.  These results signify no significant structural perturbations upon binding of ethidium.  Hence, we conclude that ethidium does bind G-quadruplex DNA, however this binding is not inducing a significant change in the quadruplex structure.  In addition, CD studies indicate an intercalative mode of binding by the presence of an induced CD signal at 376 nm.