It is thought that an insulator must reside between the enhancer and promoter to inhibit their subsequent interactions. Insulators therefore determine the set of genes an enhancer can influence. The need for insulators arises where two adjacent genes on a chromosome have very different transcription patterns; it is critical that the inducing or repressing mechanisms of one do not interfere with the neighbouring gene.
Insulator activity is thought to occur primarily through the 3D structure of DNA mediated by the protein CTCF.
Barrier insulators prevent the spread of heterochromatin from a silenced gene to an actively transcribing gene.