Chromosomes are partitioned into distinct functional regions. For example, heterochromatin regions consist of condensed chromatin and contain few transcriptionally active genes, whereas euchromatin regions are less condensed and majority of active genes reside in the euchromatin regions. Because distinct regions reside in each chromosome, borders are accordingly established between these regions. A prevailing view of the borders is that they are 'walls' that actively inhibit communication between distinct regions on chromosomes. Although little is known about the molecular bases of these walls, specific DNA elements are considered to recruit these walls to define the positions of the borders. We call the borders established with this mechanism as 'fixed borders'. Past studies have identified various insulators (boundary DNA elements) that have been suggested to recruit fixed borders to them. Another mechanism, which we introduce and focus on in this review, does not require walls recruited by specific DNA elements at the chromosomal borders. Instead, the borders are defined by a balance of opposing enzymatic activities located at the opposite sides of the resultant borders. We name these borders 'negotiable borders'. Here we review some of the recent progress in the field that offer valuable insight into mechanisms of establishing structural and functional borders on chromosomes.
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