Cooperation of PRC2 and PRC1 in epigenetic silencing of genes. PRC2 catalyzes trimethylation of histone H3 at lysine 27 (H3K27me3) (A), which is recognized and bound by CBX proteins such as CBX7, a PRC1 subunit, to subsequently recruit PRC1 for induction of monoubiquitination of histone H2A at lysine 119 (H2AK119ub1)^7,8  (B).Conversely, recent studies show that a variant form of PRC1 can act upstream of PRC2 to initiate formation of the polycomb domain; in this case, H2AK119ub1 serves as a PRC2 recruitment mechanism (C).^12-14  In addition, EED is also shown to interact to PRC1 physically.¹⁵  CpG, cytosine guanine dinucleotide; Me, trimethylation; Ub, ubiquitination.