RNF2 (UniProt/Swiss-Prot entry Q99496) is an essential component of the PRC1 Polycomb group (PcG) protein complex. PcG proteins form multiprotein complexes that are important for the transcriptional repression of various genes involved in development and cell proliferation. The PRC1 complex. mediates monoubiquitination of ‘Lys-119’ of histone H2A (H2AK119Ub), thereby playing a central role in the histone code and in gene regulation. H2AK119Ub gives a specific tag for epigenetic transcriptional repression. It is required to maintain the transcriptionally repressive state of many genes, including Hox genes and participates in X chromosome inactivation of female mammals. PRC1 may also be involved in the initiation of both imprinted and random X inactivation.