A TAD is a region of the genome in which the DNA (or chromatin) is folded up into a bunch of loops, in which transcriptional-regulatory elements & genes interact. TADs thus facilitate long-range promoter-enhancer interactions, effectively forming genomic regulatory blocks/regions (GRBs). The boundaries of TADs may restrict the influence of enhancers. 
The region contained within a TAD may be transcriptionally active or inactive. It contains multiple DNA loops that are kept separate from neighbouring TADs by proteins such as CTCF and cohesin. Evidence now suggests that a genomic deletion can be pathogenic by removing a boundary element which then can bring a normally-inactive gene close to an active one (or vice versa), thus changing expression levels of the gene. |
