We describe here a quantitative real-time PCR assay for the detection of single-base-pair differences that does not require fluorescently labeled gene-specific probes or complicated primer combinations. Following PCR or RT-PCR of a gene segment that may contain allele-specific differences, 100 pg amplified product are used for a real-time PCR with allele-specific primers and SYBR Green. The use of HEPES buffer at a pH of 6.95 together with AmpliTaq DNA polymerase results in a threshold difference between the correct template and the mismatched template of as many as 20 cycles, depending on the mismatch. Correct matches can be detected in an excess of mismatched template at least at the 0.01 level for the six primer-template matches versus mismatches tested: GC vs. A.C, AT vs. G.T, GC vs. C.C, GC vs. G.G, AT vs. C.T, and GC vs. G.A. Because the initial amplification is separate from real-time detection, conditions can be independently optimized for each step, making the assay particularly suitable for the detection of allele-specific expression in single cells.