Polycomb: a paradigm for genome organization from one to three dimensions.

Polycomb group proteins are important transcriptional repressors in developmental control, both stably silencing genes out of their appropriate lineage, and conferring dynamic regulation of genes whose expression changes in response to developmental cues. Polycomb is a key organizer of the linear epigenome, forming distinct chromatin domains of associated histone modifications, and fine-tuning the activities of genetic elements. Polycomb also modulates three-dimensional genome architecture by the formation of regulatory chromatin loops and coalescing target genes at discrete nuclear foci. Recent studies suggest that the linear epigenetic domains and chromosome architecture are intimately linked and the developmental plasticity of these scales of chromosome organization is beginning to be explored.