Reverse transcriptase-polymerase chain reaction (RT-PCR) is a highly sensitive method that permits the detection and quantification of RNA transcripts. It is particularly useful where transcripts are in very low abundance and where the amount of starting RNA is fairly limited, such as from a protoplast transfection. The procedure is based on production of a cDNA copy of the transcript by primer extension (see Chapter 21 ) and PCR amplification of the cDNA transcript by Taq DNA polymerase. The ability to amplify cDNA transcripts by Taq polymerase gives the flexibility and sensitivity, found with standard DNA PCR amplification, in the analysis of RNAs.