Fig. 7. Model for mechanism of XCds1 inactivation at meiotic G₂/M-phase transition in Xenopus oocytes. Although it remains unclear whether XCds1 is actually involved in G₂-phase arrest of immature oocytes (dashed line), progesterone might overcome or bypass its effect to induce the initial activation of cyclin B-Cdc2 kinase, leading to the downregulation of XCds1. The inactivation of XCds1 would presumably be accomplished through phosphorylation of Xenopus functional homolog of BRCT protein by cyclin B-Cdc2 kinase.