RINGO was identified as a Cdc2-binding and activating protein which is necessary and sufficient to trigger G2/M progression in Xenopus oocytes. We have investigated whether the function of RINGO is conserved in mouse oocytes. We show that RINGO induces Germinal Vesicle BreakDown (GBVD) in mouse oocytes. Mos is known to induce GVBD in mouse oocytes, and is also involved in the metaphase II arrest, which is due to the CSF (CytoStatic Factor) activity. We found that RINGO also has CSF activity and induces cleavage arrest after injection into one blastomere of a late two-cell mouse embryo, like Mos. However, RINGO also inhibits polar body extrusion of wild type mouse oocytes. The same effect of RINGO on first and second polar body extrusion was observed in Mos -/- mouse oocytes. The injection of RINGO mimics Mos effects: GVBD induction and efficient cleavage arrest. However, our results in mouse oocytes suggest that RINGO may have additional functions in meiosis regulation.