(A) Layer of daunomycin molecules in complex with the tetramolecular parallel G-quadruplex o-tel6 of sequence d(TGGGGT), as obtained by X-ray crystallography [76]. (B,C) Different layers of daunomycin molecules in complex with the tetramolecular parallel G-quadruplex o-tel4 of sequence d(GGGG), as obtained by X-ray crystallography [77]. (D) Crystal structure of the complex between the tetramolecular parallel G-quadruplex o-tel4 of sequence d(GGGG) and daunomycin [77]. G-quadruplex units are 5′-5′ stacked. (E) NMR structure of the complex between the tetramolecular parallel G-quadruplex o-tel6 of sequence d(TGGGGT) and distamycin [78]. 5′- and 3′-ends of the G-quadruplex are at the top and bottom, respectively. Adapted with permission from refs. [76,77,78], Copyright 2003 American Chemical Society and Copyright 2012 Oxford University Press and Copyright 2007 American Chemical Society.