RING1B is a E3 ubiquitin-protein ligase that mediates monoubiquitination of 'Lys-119' of histone H2A (H2AK119Ub), thereby playing a central role in histone code and gene regulation. H2AK119Ub gives a specific tag for epigenetic transcriptional repression and participates in X chromosome inactivation of female mammals. It may be involved in the initiation of both imprinted and random X inactivation. It E3 ubiquitin-protein ligase activity is enhanced by BMI1/PCGF4.
