Once Pol II has transcribed about 200 nucleotides from the transcription start site, elongation through most genes is highly processive. Chromatin immunoprecipitation with antibody to Pol II, however, indicates that the amount of Pol II at various positions in a transcription unit in a population of cells varies greatly (see Figure 9-18b, right). This finding indicates that the enzyme can elongate through some regions much more rapidly than others. In most cases, Pol II does not terminate transcription until after a sequence is transcribed that directs cleavage and polyadenylation of the RNA at the sequence that forms the 3′ end of the encoded mRNA. Pol II can then terminate transcription at any of multiple sites located 0.5–2 kb beyond this poly(A) addition site. Experiments with mutant genes show that termination is coupled to the process that cleaves and polyadenylates the 3′ end of a transcript, which is discussed in the next chapter.