Insulator regulation of the expression of T-cell receptors. With DNA looping over long distances, binding at enhancers could activate the wrong promoter. The insulator confines the action of enhancers to their matching promoter. In the regulatory regions of the genes shown here, for the α and δ chains of the T-cell receptor, the insulator prevents activation of the α promoter by the δ enhancer, and of the δ promoter by the α enhancer. CTC-binding factor (CTCF) binds to insulators, but how it functions is still uncertain.