Regardless of its position within the DNA film, cross-linked daunomycin (DM) is efficiently reduced electrochemically, indicating that the electron transfer exhibits a shallow distance dependence. Upon the introduction of an intervening cytosine–adenine (CA) mismatch, the electrochemical response is dramatically attenuated (shown schematically). Therefore, the DNA double helix can facilitate long-range electron transfer, but only in the presence of a well-stacked pathway.