In the June issue of Nature Cell Biology, D. Landiera et al. from the Imperial College School of Medicine (London, UK) reported in a letter their study results on identifying a novel subunit of the polycomb repressor complex 2 (PRC2), Jarid2, which regulates development/differentiation of mouse embryonic stem cells (ESCs). The investigators found that in Jarid2-deficient ESCs there were reduced histone methylation (HEK4me2/me3 and H3K27 me3) which mark recruitment of PRC1/PRC2. Additionally, lost of Jarid2 resulted in a reduced presence of phosphorlyated (Ser5P) RNA polymerase II (RNAP) at the target genes. This observation suggests that Jarid2 has role in also recruiting RNAP to the bivalent genes. The researchers also found ESCs lacking Jarid2 were severely compromised in their ability to differentiate into neural and mesodermal lineages. These mutant cells lack the ability to activated lineage-specific gene expression. The authors concluded from their observation "that transcriptional priming of bivalent genes in pluripotent ES cells is Jarid2-dependent, and suggests that priming is critical for subsequent multi-lineage differentiation."