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The Polycomb group (PcG) proteins have an important role in controlling the expression of genes essential for development, differentiation and maintenance of cell fates (1,2).The Polycomb repressive complex 2 (PRC2) is believed to regulate transcriptional repression by catalysing the di- and tri-methylation of lysine 27 on histone H3 (H3K27me2/3) (2). At present, it is unknown how the PcG proteins are recruited to their target promoters in mammalian cells (3). Here we show that PRC2 forms a stable complex with the Jumonjiand ARID-domain-containing protein, JARID2 (ref. 4). Using genome-wide location analysis, we show that JARID2 binds to more than 90% of previously mapped PcG target genes. Notably, we show that JARID2 is sufficient to recruit PcG proteins to a heterologous promoter, and that inhibition of JARID2 expression leads to a major loss of PcG binding and to a reduction of H3K27me3 levels on target genes. Consistent with an essential role for PcG proteins in early development (5-8), we demonstrate that JARID2 is required for the differentiation of mouse embryonic stem cells. Thus, these results demonstrate that JARID2 is essential for the binding of PcG proteins to target genes and, consistent with this, for the proper differentiation of embryonic stem cells and normal development., PcG proteins are essential for embryonic development (5-8). They execute their repressive functions in two distinct multiprotein complexes named PRC1 and PRC2 (refs 2, 3) and share a large number [...]