In the loop: long range chromatin interactions and gene regulation.

Enhancers, silencer and insulators are DNA elements that play central roles in regulation of the genome that are crucial for development and differentiation. In metazoans, these elements are often separated from target genes by distances that can reach 100  Kb. How regulation can be accomplished over long distances has long been intriguing. Current data indicate that although the mechanisms by which these diverse regulatory elements affect gene transcription may vary, an underlying feature is the establishment of close contacts or chromatin loops. With the generalization of this principle, new questions emerge, such as how the close contacts are formed and stabilized and, importantly, how they contribute to the regulation of transcriptional output at target genes. This review will concentrate on examples where a functional role and a mechanistic understanding has been explored for loops formed between genes and their regulatory elements or among the elements themselves.