Figure 1.

Key features of 3D genome organization. (A) Spatial chromosome folding in the nucleus separates the genome into two compartments: the A compartment that mainly consists of active genes and euchromatin (yellow spheres), and the B compartment that is enriched for inactive genes and heterochromatin (blue spheres). While regions of the A compartment localize to the nuclear interior, B compartment domains reside near the nuclear lamina. (B) A/B compartments contain megabase-sized self-interacting regions called topologically associating domains (TADs, depicted here as individual spheres). TADs tend to have a uniform compartment association and are separated by border regions. (C) TADs are considered functional units of chromosome organization in which enhancers (shown as boxes marked with ‘E’) interact with genes or other REs to control transcription. TAD border insulation is thought to restrict the search space of enhancers and promoters and to prevent unwanted regulatory contacts to be formed.