We present a simple and novel assay-employing a universal molecular beacon (MB) in the presence of Hg(2+)-for the detection of single nucleotide polymorphisms (SNPs) based on Hg(2+)-DNA complexes inducing a conformational change in the MB. The MB (T(7)-MB) contains a 19-mer loop and a stem of a pair of seven thymidine (T) bases, a carboxyfluorescein (FAM) unit at the 5'-end, and a 4-([4-(dimethylamino)phenyl]azo)benzoic acid (DABCYL) unit at the 3'-end. Upon formation of Hg(2+)-T(7)-MB complexes through T-Hg(2+)-T bonding, the conformation of T(7)-MB changes from a random coil to a folded structure, leading to a decreased distance between the FAM and DABCYL units and, hence, increased efficiency of fluorescence resonance energy transfer (FRET) between the FAM and DABCYL units, resulting in decreased fluorescence intensity of the MB. In the presence of complementary DNA, double-stranded DNA complexes form (instead of the Hg(2+)-T(7)-MB complexes), with FRET between the FAM and DABCYL units occurring to a lesser extent than in the folded structure. Under the optimal conditions (20 nM T(7)-MB, 20 mM NaCl, 1.0 muM Hg(2+), 5.0 mM phosphate buffer solution, pH 7.4), the linear plot of the fluorescence intensity against the concentration of perfectly matched DNA was linear over the range 2-30 nM (R(2) = 0.991), with a limit of detection of 0.5 nM at a signal-to-noise ratio of 3. This new probe provides higher selectivity toward DNA than that exhibited by conventional MBs.