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The Polycomb group proteins foster gene repression profiles required for proper development and unimpaired adulthood, and comprise the components of the Polycomb-Repressive Complex 2 (PRC2) including the histone H3 Lys 27 (H3K27) methyltransferase Ezh2. How mammalian PRC2 accesses chromatin is unclear. We found that Jarid2 associates with PRC2 and stimulates its enzymatic activity in vitro. Jarid2 contains a Jumonji C domain, but is devoid of detectable histone demethylase activity. Instead, its artificial recruitment to a promoter in vivo resulted in corecruitment of PRC2 with resultant increased levels of di- and trimethylation of H3K27 (H3K27me2/3). Jarid2 colocalizes with Ezh2 and MTF2, a homolog of Drosophila Pcl, at endogenous genes in embryonic stem (ES) cells. Jarid2 can bind DNA and its recruitment in ES cells is interdependent with that of PRC2, as Jarid2 knockdown reduced PRC2 at its target promoters, and ES cells devoid of the PRC2 component EED are deficient in Jarid2 promoter access. In addition to the well-documented defects in embryonic viability upon down-regulation of Jarid2, ES cell differentiation is impaired, as is Oct4 silencing.