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Uterine stromal cell decidualization is a dynamic physiological process in which cell proliferation, differentiation and apoptosis are orchestrated and occur in a temporal and cell-specific manner. This process is important for successful embryo implantation. Many cell-cycle regulators are involved in decidualization. The protein cyclin G1 is a unique regulator of the cell cycle with dual functions in cell proliferation. It was reported that cyclin G1 is expressed in mouse uterine stromal cells during the period of peri-implantation. To prove the function of cyclin G1 in mouse uterine stromal cells during this period, immunohistochemistry was used to stain mouse uterine tissues on days 4-8 of pregnancy. The results showed obvious spatial and temporal expression of cyclin G1 in uterine stromal cells, and that it is expressed in the cells of the primary decidual zone (PDZ) on day 5 and secondary decidual zone (SDZ) on days 6 and 7, when the stromal cells experienced active proliferation and differentiation was initiated. Applying the decidualization model of cultured primary stromal cells in vitro, we further revealed that the expression of cyclin G1 is associated with decidualization of stromal cells induced by medroxyprogesterone acetate (MPA) and estradiol-17β (E2). RNA interference was used for the knockdown of cyclin G1 in the induced decidual cells. Flow cytometry analysis indicated that the proportion of cells in the S stage was increased, and decreased in the G2/M phase. Our study indicates that cyclin G1, as a negative regulator of the cell cycle, plays an important role in the process of decidualization in mouse uterine stromal cells by inhibiting cell-cycle progression.