Primer extension offers an alternative to a nuclease protection assay (S1 nuclease mapping) for quantifying and mapping RNA transcripts. The hybridization probe for primer extension is a synthesized oligonucleotide, whereas S1 mapping requires isolation of a DNA fragment. Both methods provide information where a mRNA starts and provide an estimate of the concentration of a transcript by the intensity of the transcript band on the resulting autoradiograph. Unlike S1 mapping, however, primer extension can only be used to locate the 5’-end of an mRNA transcript because the DNA synthesis required for the assay relies on reverse transcriptase (only polymerizes in the 5’ → 3’ direction).
