Abstract. Rat uterine sensitization-associated gene-1 (USAG-1) mRNA is expressed in the uterus during the peri-implantation period, and its mRNA expression in uterine epithelial cells is highest on day 5 of pregnancy. On the other hand, since changes in USAG-1 mRNA expression in the mouse uterus are not seen during the estrous cycle, USAG-1 expression might be specifically regulated by embryonic factors rather than by the maternal environment. However, the expression pattern and function of USAG-1 in the mouse uterus have not been determined. Thus, we examined the tissuespecific USAG-1 mRNA expression in the uteri of ICR mice during peri-implantation using real-time quantitative PCR. Uterine tissues, such as the myometrium, luminal epithelium, and stroma, were collected by laser capture microdissection at 3.5-6.5 dpc. USAG-1 mRNA was expressed in the uteri of pregnant mice from 3.5 dpc to 6.5 dpc, and the highest level of expression was seen at 4.5 dpc (P<0.01). Significantly high USAG-1 mRNA expression was detected in the luminal epithelium at 4.5 dpc (P<0.05). The stroma and myometrium exhibited unchanged expression levels of USAG-1 mRNA at 3.5-5.5 dpc. USAG-1 mRNA was undetectable in blastocysts and implanting embryos. Expression of USAG-1 mRNA appears to be associated with blastocyst implantation to the luminal epithelium, suggesting that physiological or biochemical contact of the blastocyst to the uterus is required for USAG-1 expression. Key words: Implantation, Mouse, mRNA, Uterine sensitization-associated gene-1 (USAG-1), Uterus (J. Reprod. Dev. 53: [931][932][933][934][935][936] 2007) or successful establishment of pregnancy, blastocyst development must proceed beyond the attachment stage and result in endometrial decidualization and subsequent placental formation. Estrogen and progesterone regulate the events that lead to the receptivity and sensitization of the endometrium for the decidual cell reaction [1]. In rodents and humans, implantation is i n i t i a t e d w i t h a n a p p o s i t i o n b e t w e e n t h e trophoblast and apical surface of the luminal epithelium. This is followed rapidly by adhesion of the conceptus to the luminal surface and then penetration through the luminal epithelium to the underlying stroma, which initiates the endometrial d e c i d u a l i z a t i o n r e a c t i o n [ 2 , 3 ] . U t e r i n e sensitization-associated gene-1 (USAG-1), a gene of unknown function during conceptus development, is upregulated in the sensitized endometrium of the rat uterus during the peri-implantation period [4].