Polycomb proteins, known to be important in development, repress many developmental regulators in mammalian embryonic stem (ES) cells until these genes are ready to be turned on during differentiation, according to two new studies in this week?s Nature and Cell. A third study, published in Cell, shows that a characteristic pattern of histone methylation in ES cells also represses genes involved in cell differentiation. Understanding epigenetic differences between stem cells and differentiated cells may help researchers exert more control over ES cell differentiation, the authors suggest.The repressive mechanisms discovered in ES cells differ from those seen in somatic cells, because repression must be reversible for cells to remain pluripotent, said Vincenzo Pirrotta of Rutgers University in Piscataway, NJ, who was not involved in the studies. ?Somehow these genes, although repressed, are poised to be expressed again,? Pirrotta told The Scientist. ?Given the right signal, they can take...

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