Colocalization of like-regulated genes and specialized transcription factories. (a) Quadruple-label RNA immuno-FISH of three genes that are being transcribed and their association with RNAPII transcription factories. RNAPII staining is shown on the left and an overlay of the RNAPII staining showing the contributions of the genes is on the right. The side panels show the enlarged images of colocalizing FISH signals, showing that transcription factories can simultaneously transcribe at least three genes, located on different chromosomes. (b) Immunofluorescence detection of Klf1 (red) and RNAPII transcription factories (green), showing the selective and specialized nature of transcription factories. (c) Triple-label RNA immuno-FISH for Hbb and Epb4.9, showing association of these genes at Klf1 foci. All images show definitive erythroid cells and the scale bar in each panel represents 2 μm. Reproduced, with permission, from .