Posttranslational modifications of histones play important roles in the regulation of chromatin structure and function. Histone H2A ubiquitination is a predominant modification important for a variety of cellular processes. We have previously discovered that Polycomb Repressive Complex 1 (PRC1), a fundamental developmental regulator, acts as a ubiquitin ligase for H2AK119 ubiquitination (H2AK119ub). This study links H2AK119ub to PRC1-mediated gene silencing of key developmental genes and the essential roles of PRC1 in cell identity, tumorigenesis, and genomic imprinting. Several proteins have been shown to bind H2AK119ub; however, how this modification elicits downstream gene silencing events remains largely obscure. We recently identified Remodeling and Spacing Factor 1 (RSF1) as a novel H2AK119ub-binding protein, providing a gateway to dissect the mechanism of action of H2AK119ub. We discovered that RSF1 binds H2AK119ub through a previously uncharacterized region designated as the ubiquitinated H2A binding (UAB) motif, and that RSF1 is required both for silencing of H2AK119ub target genes and for maintaining the normal H2AK119ub nucleosome pattern at promoter regions. We further demonstrated that, during Xenopus early embryonic development, RSF1 regulates mesodermal cell specification and gastrulation in a fashion similar to Ring1, a Xenopus PRC1 subunit mediating H2AK119ub. Although these studies reveal that RSF1, as a H2AK119ub-binding protein, is required for H2AK119ub target gene repression, it remains to be established that reading the H2AK119ub mark is the mechanism by which RSF1 represses gene expression. Here, we propose a series of structural, in vitro, and in vivo studies to delineate the mechanism of action of RSF1 in H2AK119ub function. We hypothesize that RSF1 is a key reader of H2AK119 ubiquitination that mediates its roles in gene silencing, chromatin remodeling, and development. We propose to address three questions critical to this hypothesis: 1) how does RSF1 specifically recognize H2AK119ub nucleosomes? 2) how does RSF1 modulate the organization of H2AK119ub chromatin? and 3) what is the significance of RSF1 in H2AK119ub-regulated physiological processes? Given the fundamental roles of PRC1 and RSF1 in cell fate determination during normal development and the extensive involvement of PRC1 and RSF1 in cancer development, these studies will significantly advance our understanding of the epigenetic mechanisms controlling normal development as well as pathogenic processes.