| Summary: | Originally two types of Polycomb Repressive Complexes (PRCs) were described, canonical PRC1 (cPRC1) and PRC2. Recently, a versatile set of complexes were identified and brought up several dilemmas in PRC mediated repression. These new class of complexes were named as non-canonical PRC1s (ncPRC1s). Both cPRC1s and ncPRC1s contain Ring finger protein (RING1, RNF2) and Polycomb group ring finger catalytic (PCGF) core, but in ncPRCs, RING and YY1 binding protein (RYBP), or YY1 associated factor 2 (YAF2), replaces the Chromobox (CBX) and Polyhomeotic (PHC) subunits found in cPRC1s. Additionally, ncPRC1 subunits can associate with versatile accessory proteins, which determine their functional specificity. Homozygous null mutations of the ncPRC members in mice are often lethal or cause infertility, which underlines their essential functions in mammalian development. In this review, we summarize the mouse knockout phenotypes of subunits of the six major ncPRCs. We highlight several aspects of their discovery from fly to mice and emerging role in target recognition, embryogenesis and cell-fate decision making. We gathered data from stem cell mediated in vitro differentiation assays and genetically engineered mouse models. Accumulating evidence suggests that ncPRC1s play profound role in mammalian embryogenesis by regulating gene expression during lineage specification of pluripotent stem cells.
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