Fig. 4. A model for transcript production (Iborra et al., 2004). (A) The CTD has the potential to associate with sites involved in capping, transcript degradation, translational proofreading (involving the translational and NMD machineries), proteolysis, splicing and polyadenylation. It remains unclear whether all bind to the CTD simultaneously, or whether they attach and detach as needed. [Wetterberg et al. (Wetterberg et al., 2001) have analysed the association of the splicing machinery during the synthesis of an exceptionally long mRNA.] (B) Transcription began as the template bound to the polymerizing complex and was reeled in as the transcript was extruded; the CTD is now hyper-phosphorylated (CTD^P), and a cap has been added. (C) The transcript continues to be extruded through a splicing site as the ribosome/NMD machinery begins proofreading the now-spliced message (and so does not read introns, which might contain many termination codons). (D) Once introns are removed (lariat), the transcript is cleaved and polyadenylated, and is ready to leave for the cytoplasm. If errors are detected, the faulty transcript and peptide produced during proofreading are degraded by nucleases and proteasomes.