Mg2+ is one of the most important cations in cells affecting the structures and functions of the proteins and nucleic acids It should be noted that Mg2+ is indispensable in DNA transcription where G-quadruplex is believed to be actively involved Therefore it is Important to investigate the influence of Mg2+ G-quadruplex Here we studied the effect of Mg2+ G-quadruplex DNA with CD FRET EMSA and PCR-stop assay We found that various G-quadruplexes could be differentiated through simultaneous addition of both K+ and Mg2+ which could be used for selective identification of G-quadruplexes in promoter oncogene but not in telomere Mg2+ physiological relevant concentration not only greatly enhanced the thermostat:nifty of oncogene G-quadruplexes but also efficiently protected them from unfolding by their complementary strands which revealed the great impact of Mg2+ on the equilibrium between promoter G-quadruplex and duplex DNA The PCR-stop assay further confirmed that Mg2+ could affect gene transcription by stabilizing promoter G-quadruplex The above studies were carried out for various G-quadruplexes of varying sequences in promoter oncogenes and telomeric region Our results suggest that Mg2+ may be a key regulator for G-quadruplexes of oncogene promoter which can subsequently affect the expression of related genes (C) 2010 Elsevier Inc All rights reserved