Figure 1.

Mode of insulator action. (A) One kind of insulator (I) can block the propagation of a heterochromatic, silencing domain into a region containing transcriptionally active genes. (B) Other ‘enhancer blocking’ insulator elements can prevent the action of an enhancer (Enh1) in gene system 1 from activating a promoter (Pro2) in gene system 2, without inhibiting the ability of Enh1 to activate Pro1. (C) A test for the enhancer blocking activity of CTCF (I) is to place it between an enhancer and promoter, which blocks enhancer action (top) or outside the enhancer-promoter domain, where it has no inhibitory action (bottom).