Single-stranded signal primers are modified by linkage to two dyes which form a donor/acceptor dye pair. The two dyes are positioned in sufficiently close spatial proximity on the signal primer that the fluorescence of the first dye is quenched by the second dye. The signal primer may further comprise a restriction endonuclease recognition site (RERS) between the two dyes. As the signal primer is initially single-stranded and remains single-stranded in the absence of target, the restriction endonuclease recognition site is not cleavable or nickable by the restriction endonuclease. In the presence of target, however, signal primer and the restriction endonuclease recognition site are rendered double-stranded and cleavable or nickable by the restriction endonuclease. Cleavage or nicking separates the two dyes and a change in fluorescence due to decreased quenching is detected as an indication of the presence of the target sequence or of target sequence amplification.