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. Author manuscript; available in PMC: 2020 Sep 8.
Published in final edited form as: Nat Rev Mol Cell Biol. 2019 Jun 1;20(6):327–337. doi: 10.1038/s41580-019-0114-6

Figure 3. Alternative mechanisms of TAD boundary formation.

Figure 3

(A) TADs may be the result of loop extrusion by cohesin complexes (green rings). One TAD may consist of multiple loops that are dynamically formed and resolved. CTCF, when bound in the correct orientation, could act as a “road block” that stops progression of loop extrusion, thereby creating a TAD border. (B) Similarly, active genes, the transcription pre-initiation complex, or a chromatin mark associated with it, could block loop extrusion.