The RNAP occasionally stops and starts moving backwards when it encounters a roadblock or some difficult sequences. When this happens, the EC gets stuck because the reactive 3 edge of the RNA is out of the active site. The transcript cleavage factor TFS (a TFIIS homolog) helps resolve this issue by generating a cut so that a new 3 end is available in the active site. Some archaeon have up to 4 paralogs of TFS with divergent functions.