Genetic evidence for asymmetric blocking of higher-order chromatin structure by CTCF/cohesin

Dear Editor,Similar to higher-order folding of polypeptide chains into functional proteins,linear DNA molecules are spatially folded in a hierarchical and dynamic manner into three-dimensional(3D)functional chromatin structures in eukaryotic nuclei(Huang and Wu,2016;Rowley and Corces,2018).This dynamic folding is closely related to many nuclear processes such as DNA replication and repair,chromosomal translo­cation,recombination,and segregation,as well as RNA transcription,splicing,and transport.In particular,dynamic long-distance chromatin looping interactions,which result in close spatial contacts between distal enhancers and target promoters,are thought to play a role in controlling precise spatiotemporal as well as cell-type specific gene expression during animal development(Rowley and Corces,2018).Mammalian genomes contain numerous noncoding regula­tory elements that regulate these dynamic long-distance chromatin looping interactions.