Polycomb-repressive complex 1 (PRC1) has a central role in the regulation of 
heritable gene silencing during differentiation and development. PRC1 
recruitment is generally attributed to interaction of the chromodomain of the 
core protein Polycomb with trimethyl histone H3K27 (H3K27me3), catalyzed by a 
second complex, PRC2. Unexpectedly we find that RING1B, the catalytic subunit of 
PRC1, and associated monoubiquitylation of histone H2A are targeted to closely 
overlapping sites in wild-type and PRC2-deficient mouse embryonic stem cells 
(mESCs), demonstrating an H3K27me3-independent pathway for recruitment of PRC1 
activity. We show that this pathway is mediated by RYBP-PRC1, a complex 
comprising catalytic subunits of PRC1 and the protein RYBP. RYBP-PRC1 is 
recruited to target loci in mESCs and is also involved in Xist RNA-mediated 
silencing, the latter suggesting a wider role in Polycomb silencing. We discuss 
the implications of these findings for understanding recruitment and function of 
Polycomb repressors.