Labeling of RNA in isolated HeLa cell nuclei in vitro reveals an abundance of short RNA chains made by RNA polymerase II. These short chains were initiated prior to isolation of the nuclei. The short abundant chains are increased in amount in nuclei isolated from cells treated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Kinetic evidence indicates that the bulk of the putative heterogeneous nuclear RNA (hnRNA) precursor molecules that are terminated early in vivo are terminated approximately 100-300 nucleotides from sites of initiation. DRB increases the frequency of early termination, but there is a fraction of hnRNA precursor molecules whose elongation is not affected by DRB. Heparin is useful in studies of hnRNA transcription in isolated nuclei because it enhances chain elongation.

Tamm, I., & Kikuchi, T. (1979). Early termination of heterogeneous nuclear RNA transcripts in mammalian cells: accentuation by 5,6-dichloro 1-beta-D-ribofuranosylbenzimidazole. Proceedings of the National Academy of Sciences, 76(11), 5750–5754.