To estimate T m,min, the PCR denaturation temperature below which PCR is not efficient it was assumed, as an initial approximation, that495% hypochromicity must be present for PCR to work (i.e. any given sequence must be completely denatured, otherwise it re-forms immediately when temperature is lowered in the reaction and inhibits primer binding). The percent melting (hypochromicity)versus-temperature relations for GC-clamp-containing PCR products and Surveyor TM activity-generated products were estimated using the POLAND algorithm (37) , and the thermodynamic parameters determined by Blake and Delcourt for 75 mM NaCl in the solution (38) were used. In order to force agreement at a single point, predicted and observed values for a p53 exon 8 sequence containing a short GC-clamp were normalized at 888C. This shift accounts for the influence on T m,min of NaCl and Mg++ content in the reaction, the presence of the SYBR-GREEN/LC-GREEN dyes and the proprietary composition of PCR buffers. The T m,min of all other PCR products was then estimated using these semi-empirically determined parameters.