This assay is based on the observation that the transcription complex will fall off from a DNA template if complex reaches the end of the template. A restriction enzyme site within the gene can therefore be chosen generate run-off transcripts of a proper length. As long as the RNA is initiated at the same place, the run-off transcripts should have an uniform length . This method eliminates the complication from the transcription elongation. The product can be assayed by direct labeling the RNA product during transcription or by other methods that can quantitate the unlabeled RNA from trasncription reaction, such as the primer extension or RNase protection assays.