Results of the DSN action on one mismatch-containing (A,B) and perfectly matched (C) DNA duplexes. Duplexes formed by 5-carboxyfluorescein (Fl)-5′-gccctatagt-3′-TAMRA signal probe and complementary targets were incubated with DSN at 35°C for 15 min. Emission spectra were obtained on the spectrofluorometer, with excitation at 480 nm. Dotted line, substrate fluorescence in the absence of enzyme; solid line, substrate fluorescence after incubation with DSN.