Example of quantitative real-time polymerase chain reaction

The polymerase chain reaction (PCR) can be quantitated in its early exponential phase. Panel A: If there are many targets in the starting sample (eg, curves on the left), the generated fluorescent signal will cross a predetermined threshold value (C_T) early in the PCR cycling process. If there are few targets (eg, curves on the right), the signal will rise more slowly and will cross C_T at a later PCR cycle. Panel B: When C_T is plotted versus the starting quantity of target DNA, the relationship is linear over more than 8 logarithms. To establish the amount of targets in a sample, one can simply determine the cycle number at which C_T is crossed and use the standard curve to infer the number of targets present in the sample. Panel C: In an example of a cancer patient with t(14;18), the initially large number of targets present (red curve) is markedly reduced after treatment (blue curve).