The E3 ubiquitin ligase activity of RING1B is not essential for early mouse development

Polycomb-repressive complex 1 (PRC1) and PRC2 maintain repression atmanydevelopmental genes inmouseembryonic stem cells and are required for early development. However, it is still unclear how they are targeted and how they function. We show that the ability of RING1B, a core component of PRC1, to ubiquitinate histone H2A is dispensable for early mouse embryonic development and muchof thegene repression activity ofPRC1. Ourdata support a model in which PRC1 and PRC2 reinforce each other's binding but suggest that the key functions of PRC1 lie beyond the enzymatic capabilities of RING1B.