Gene silencing can be achieved by packaging of genes into repressive heterochromatin domains. In the January 31 Science, two papers describe the dynamics of mammalian heterochromatin and the associated heterochromatin protein 1 (HP1).

Richard Festenstein and colleagues generated transgenic mice that express a chimeric protein of HP1β fused to green fluorescent protein (GFP) in T lymphocytes (Science, 299:719-721, January 31, 2003). HP1β-GFP was seen in heterochromatic foci in T cell nuclei. They used fluorescence recovery after photobleaching (FRAP) experiments in living T cells and showed that HP1 is highly mobile, suggesting heterochromatin maintenance is dynamic. HP1 mobility was increased upon T cell activation.

In the second paper Thierry Cheutin and colleagues describe a similar approach using GFP fusions of HP1α, -β and -γ in CHO cells (Science, 299:721-725, January 31, 2003). The GFP-HP1α and –β forms were localized in larger heterochromatin domains. Experiments in...

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