eukaryoltc cases as well, though in some cases the distances over which proteins work is very large and it is not dear how' the looping occurs.

Activation at a distance raises another problem. When bound at an enhancer, there may be several genes within range of an activator, vet a given enhancer typically regulates only one gene. Other regulatory sequences—called insulators or boundary elements—are found betwoen enhancers and some promoters. Insulalnrs block activation of the promoter by activators bound at the enhancer. These elements, although still poorly understood, ensure activators do not work indiscriminately.

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