To map transcriptional start sites (TSS), reverse transcriptase is used to convert mRNA into ssDNA. By annealing primers to the ssDNA, dsDNA can be produced towards the TSS by PCR. These strands can then be resolved on an agarose gel to determine the location of a TSS. The could also be mapped back to a reference genome by DNA sequencing.
Reverse transcriptase is used as it is able to extend only up to the TSS, before automatically stopping.