The Polycomb group (PcG) proteins have an important role in controlling the 
expression of genes essential for development, differentiation and maintenance 
of cell fates. 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). At present, it is unknown how the PcG proteins 
are recruited to their target promoters in mammalian cells. Here we show that 
PRC2 forms a stable complex with the Jumonji- and 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, 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.