To enable selective amplification of the mutationcontaining fragments in the real-time PCR step, modified primers are employed for the original amplification from genomic DNA ( Figure 1B ). The forward primer contains a region specific to the target gene and a high melting domain (GC-clamp), while the reverse primer contains a region specific to the target gene and an M13 tail (or vice versa). Following the TdT tailing reaction, the M13 primer is used for real-time PCR in conjunction with a primer that binds to the poly-A tail. The denaturation temperature of the real-time PCR reaction is lowered to enable PCR amplification only for fragments that do not contain GC-clamps. Because the PCR products that escape digestion by Surveyor TM contain GC-clamps ( Figure 1B ), these fragments do not amplify efficiently during PCR, thereby enabling selective amplification of Surveyor TM -selected fragments, i.e. an effective 'purification' of mutation-containing fragments. The subsequent closed-tube melting curve analysis enables clear separation of true mutant sequences from PCR dimers or other artifacts.