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Cover Recruitment of heterochromatin protein 1 (HP1) to an expressed transgene in mammalian cells results in the epigenetically heritable silencing of the gene. Shown here is a fluorescence in situ hybridization (FISH) analysis of 4-OHT-treated mammalian cells that have been cotransfected with a PAX3KRAB-luciferase reporter transgene and a KRAB-PAX3-HBD (estrogen receptor hormone-binding domain) fusion protein. Treatment with the synthetic estrogen 4-OHT activates the KRAB fusion protein and, through an interaction between KAP1 and the KRAB-binding domain, targets a KAP1/HP1/SETDB1 protein complexto stably silence the luciferase transgene. FISH was carried out using a luciferase probe and the cells were counterstained with DAPI to visualize condensed heterochromatin (blue). In most cells, the luciferase FISH signals merge with the regions of condensed chromatin (lighter, bluish dots). (For details, see Ayyanathan et al., p. 1855.)