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Abstract
Insulators can block an enhancer of one gene from activating a promoter on another
nearby gene. Almost all described vertebrate insulators require binding of the regulatory
protein CTCF for their activity. We show that CTCF copurifies with the nucleolar protein
nucleophosmin and both are present at insulator sites in vivo. Furthermore, exogenous
insulator sequences are tethered to the nucleolus in a CTCF-dependent manner. These
interactions, quite different from those of the gypsy insulator element in Drosophila,
may generate similar loop structures, suggesting a common theme and model for enhancer-blocking
insulator action.