text: Active RNA Pol II transcription holoenzymes can be clustered in the nucleus, in discrete sites called transcription factories. There are ~8,000 such factories in the nucleoplasm of a HeLa cell, but only 100–300 RNAP II foci per nucleus in erythroid cells, as in many other tissue types. The number of transcription factories in tissues is far more restricted than indicated by previous estimates from cultured cells.  As an active transcription unit is usually associated with only one Pol II holoenzyme, a polymerase II factory may contain on average ~8 holoenzymes. Colocalization of transcribed genes has not been observed when using cultured fibroblast-like cells. Differentiated or committed tissue types have a limited number of available transcription sites. Estimates show that erythroid cells express at least 4,000 genes, so many genes are obliged to seek out and share the same factory.
  The intranuclear position of many genes is correlated with their activity state. During transcription in vivo, distal active genes are dynamically organized into shared nuclear subcompartments and colocalize to the same transcription factory at high frequencies. Movement into or out of these factories results in activation (On) or abatement (Off) of transcription, rather than by recruiting and assembling a transcription complex. Usually, genes migrate to preassembled factories for transcription.
  An expressed gene is preferentially located outside of its chromosome territory, but a closely linked, inactive gene is located inside.