A DNA sequence that contributes to limitation of chromatin activation to distinct segments. In the domain boundary model, the insulator putatively binds nucleoprotein complexes along the chromatin strand and, by drawing together these boundary complexes, simultaneously condenses the chromatin and physically separates enhancer and promoter sequences, thus preventing activation of genes in this region. In the transcriptional decoy model, the insulator is imagined to provides a site for assembly of a decoy complex with which the enhancer will interact in preference to, or in competition with, the true promoter