A separation-free single-base extension (SBE) assay utilizing fluorescence resonance energy transfer (FRET) was developed for rapid and convenient interrogation of DNA methylation status at specific cytosine and guanine dinucleotide sites. In this assay, the SBE was performed in a tube using an allele-specific oligonucleotide primer (i.e., extension primer) labeled with Cy3 as a FRET donor fluorophore at the 5 '-end, a nucleotide terminator (dideoxynucleotide triphosphate) labeled with Cy5 as a FRET acceptor, a PCR amplicon derived from bisulfite-converted genomic DNA, and a DNA polymerase. A single base-extended primer (i.e., SBE product) that was 5 '-Cy3- and 3 '-Cy5-tagged was formed by incorporation of the Cy5-labeled terminator into the 3 '-end of the extension primer, but only if the terminator added was complementary to the target nucleotide. The resulting SBE product brought the Cy3 donor and the Cy5 acceptor into close proximity. Illumination of the Cy3 donor resulted in successful FRET and excitation of the Cy5 acceptor, generating fluorescence emission from the acceptor. The capacity of the developed assay to discriminate as low as 10% methylation from a mixture of methylated and unmethylated DNA was demonstrated at multiple cytosine and guanine dinucleotide sites.