Insulators define independent genomic regions that are protected from the influence of distant regulatory sequences. In the 1 November EMBO Journal, Parnell and Geyer describe a novel application of the FLP recombinase to investigate the properties of two Drosophila insulators, called gypsy and scs (EMBO J 2000, 19:5864-5874). Neither insulator affected FLP recombination and protein-protein interactions at adjacent recombination sites, suggesting that gypsy and scs do not act by general inhibition or chromatin structure rearrangement. The gypsy insulator could, however, block enhancer activity on extrachromosomal episomes created by FLP recombination, whereas the scs insulator acted as a gene silencer. So gypsy and scs appear to insulate genomic domains by two different enhancer-blocking mechanisms, which involve specific interference rather than global repression.

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