Insulators have two main functions:
# Enhancer-blocking insulators prevent distal enhancers from acting on the promoter of neighbouring genes
# Barrier insulators prevent silencing of euchromatin by the spread of neighbouring heterochromatin
While enhancer-blocking is classified as an inter-chromosomal interaction, acting as a barrier is classified as an intra-chromosomal interaction. 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. Insulators have also been found to cluster at the boundaries of topologically associating domains (TADs) and may have a role in partitioning the genome into "chromosome neighborhoods" - genomic regions within which regulation occurs.
Some insulators can act as both enhancer blocker and barriers, and some just have one of the two functions. Some examples of different insulators are:
* Drosophila melanogaster insulators gypsy and scs scs are both enhancer-blocking insulators
* Gallus gallus have insulators, Lys 5 A that have both enhancer-blocking and barrier activity, as well as HS4' that have only enhancer-blocking activity
* Saccharomyces cerevisiae insulators STAR and UAS are both barrier insulators
* Homo sapiens HS5 insulator acts as an enhancer-blocker