RNA polymerase II transcription complexes are susceptible to transcriptional stalling and arrest, when extending nascent transcripts to 30-nt. This susceptibility depends on presence on down-stream DNA, the particular DNA-sequence of the template and presence of transcription factors. Transcription factor TFIIH remains associated to the RNA pol II elongation complex until position +30. At this stage transcription elongation factor TFIIS can rescue stalled transcription elongation complexes. The transcription bubble varies between 13- and 22-nt in size.