Active genes dynamically colocalize to shared sites of ongoing transcription. The study shows that during transcription in vivo, distal genes frequently colocalize at the same transcription factory. Active genes are dynamically organized into shared nuclear subcompartments, and movement into or out of these factories results in activation or abatement of transcription. This suggests that active genes migrate to preassembled transcription sites rather than recruiting and assembling transcription complexes. The activity state of a gene is often correlated with its intranuclear localization. Transcriptionally active genes have been proposed to associate with transcription factories, discrete nuclear sites of nascent RNA production. Calculations suggest that several active genes could occupy the same factory. Using 3D fluorescence in situ hybridization (FISH), immunofluorescence, and chromosome conformation capture (3C) assays, the study assessed the spatial organization of several genes in a 40-Mb region of distal mouse chromosome 7. Results showed that transcriptionally active genes frequently colocalize with Hbb-b1. The study also found that transcribed genes are more likely to be located outside their chromosome territories, while inactive genes are more often inside. The number of RNAP II foci in erythroid cells was lower than in fibroblast-like cell lines, suggesting that transcription factories are limited in vivo. The study used 3C analysis to confirm the colocalization of transcribed alleles. It showed that distal active genes are frequently proximal to Hbb in erythroid cells. The findings suggest that genes with transcriptional potential are preferentially located outside chromosome territories, but this alone is not sufficient for transcription. The study also highlights the importance of assessing the transcribed subpopulation of alleles to observe colocalization. The study concludes that most genes move in and out of transcription factories, resulting in activation and abatement of transcription. Active genes are recruited to RNAP II compartments rather than recruiting RNAP II. The findings support the concept that genes are dynamically recruited to transcription factories, and that transcriptional activity is influenced by the dynamic association of genes with transcription factories.Active genes dynamically colocalize to shared sites of ongoing transcription. The study shows that during transcription in vivo, distal genes frequently colocalize at the same transcription factory. Active genes are dynamically organized into shared nuclear subcompartments, and movement into or out of these factories results in activation or abatement of transcription. This suggests that active genes migrate to preassembled transcription sites rather than recruiting and assembling transcription complexes. The activity state of a gene is often correlated with its intranuclear localization. Transcriptionally active genes have been proposed to associate with transcription factories, discrete nuclear sites of nascent RNA production. Calculations suggest that several active genes could occupy the same factory. Using 3D fluorescence in situ hybridization (FISH), immunofluorescence, and chromosome conformation capture (3C) assays, the study assessed the spatial organization of several genes in a 40-Mb region of distal mouse chromosome 7. Results showed that transcriptionally active genes frequently colocalize with Hbb-b1. The study also found that transcribed genes are more likely to be located outside their chromosome territories, while inactive genes are more often inside. The number of RNAP II foci in erythroid cells was lower than in fibroblast-like cell lines, suggesting that transcription factories are limited in vivo. The study used 3C analysis to confirm the colocalization of transcribed alleles. It showed that distal active genes are frequently proximal to Hbb in erythroid cells. The findings suggest that genes with transcriptional potential are preferentially located outside chromosome territories, but this alone is not sufficient for transcription. The study also highlights the importance of assessing the transcribed subpopulation of alleles to observe colocalization. The study concludes that most genes move in and out of transcription factories, resulting in activation and abatement of transcription. Active genes are recruited to RNAP II compartments rather than recruiting RNAP II. The findings support the concept that genes are dynamically recruited to transcription factories, and that transcriptional activity is influenced by the dynamic association of genes with transcription factories.