In mammals, sex differentiation of primordial germ cells (PGCs) is determined by extrinsic cues from the environment1. In female PGCs, expression of Stimulated by retinoic acid 8 (Stra8) and meiosis are induced in response to retinoic acid (RA) provided by the mesonephroi2-4. Given the widespread role of RA signaling during development8,9, the molecular mechanism specifying the competence of PGCs to timely express Stra8 and enter meiosis are unknown2,10. Here we identify gene dosage dependent roles in PGC development for Ring1 and Rnf2, two central components of the Polycomb Repressive Complex 1 (PRC1)11,13. Both paralogs are essential for PGC development between day 10.5 and 11.5 of gestation. Rnf2 is subsequently required in female PGCs for maintaining high levels of Oct4 and Nanog expression6, and for preventing premature induction of meiotic gene expression and entry into meiotic prophase. Chemical inhibition of RA signaling partially suppresses precocious Oct4 down-regulation and Stra8 activation in Rnf2-deficient female PGCs. Chromatin immunoprecipitation analyses show that Stra8 is a direct target of PRC1 and PRC2 in PGCs. These data demonstrate the importance of PRC1 gene dosage in PGC development and in coordinating the timing of sex differentiation of female PGCs by antagonizing extrinsic RA signaling. Gene expression of mouse primordial germ cells was analysed using Affymetrix microarray. Primodial germ cells were purified from embryos carrying Oct4(deltaPE)-GFP transgene by FACS.