['To examine the spatiotemporal expression of CD9 protein in the peri-implantation mouse uterus as well as the effects of ovarian steroid hormones on CD9.', 'Experimental animal study.', 'Reproductive biological center of Chinese Academy of Sciences.', 'Female Kunming white strain mice (6-8 weeks old).', 'Subcutaneous injection of P(4)/E(2); uterine tissues were collected at different times after injection.', 'The levels of protein and mRNA were detected in mouse uterus during peri-implantation and after steroid hormones treatment.', 'CD9 protein was expressed intensely in the stromal cells on days 1 and 2 of pregnancy. On days 3 and 4, the glandular and luminal epithelial cells exhibited accumulation of CD9 protein. After the initial attachment reaction on day 5, luminal epithelial and stromal cells immediately surrounding the blastocysts exhibited distinct accumulation of CD9. On days 6-8, the accumulation of CD9 occurred in decidual cells. Using ovariectomized mice, we also observed that both progesterone and estrogen upregulated uterine CD9 expression.'] Is the expression of CD9 in the peri-implantation mouse uterus upregulated in an ovarian steroid hormone-dependent manner?