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Nuclear "run-on" (or "run-off") transcription assays have been used to obtain quantitative information about the relative rates of transcription of different genes in nuclei isolated from a particular tissue or organ. This information can then be used in analyzing the factors that control the expression of specific genes. These assays are complimentary to measurements of the levels of mRNA species present in particular tissues or organs by hybridization to specific nucleic acid probes (seeChapter 18). Hybridization assays give a measure of how much mRNA is present; this is termed a "steady-state" level, since it reflects the balance between mRNA synthesis and degradation (although, of course, the "steady-state" level itself can change with time), whereas transcription assays give a direct measure of the activity of genes. [ABSTRACT FROM AUTHOR]