Hoxa-10 is an AbdominalB-like homeobox gene that is expressed in the developing genitourinary tract during embryogenesis and in the adult uterus during early pregnancy. Null mutation of Hoxa-10 in the mouse causes both male and female infertility. Defective implantation and decidualization resulting from the loss of maternal Hoxa-10 function in uterine stromal cells is the cause of female infertility. However, the mechanisms by which Hoxa-10 regulates these uterine events are unknown. We have identified two potential mechanisms for these uterine defects in Hoxa-10(-/-) mice. First, two PGE2 receptor subtypes, EP3 and EP4, are aberrantly expressed in the uterine stroma in Hoxa-10(-/-) mice, while expression of several other genes in the stroma (TIMP-2, MMP-2, ER, and PR) and epithelium (LIF, HB-EGF, Ar, and COX-1) are unaffected before implantation. Further, EP3 and EP4 are inappropriately regulated by progesterone (P4) in the absence of Hoxa-10, while PR, Hoxa-11 and c-myc, three other P4-responsive genes respond normally. These results suggest that Hoxa-10 specifically mediates P4 regulation of EP3 and EP4 in the uterine stroma. Second, since Hox genes are implicated in local cell proliferation, we also examined steroid-responsive uterine cell proliferation in Hoxa-10(-/-) mice. Stromal cell proliferation in mutant mice in response to P4 and 17β-estradiol (E2) was significantly reduced, while epithelial cell proliferation was normal in response to E2. These results suggest that stromal cell responsiveness to P4 with respect to cell proliferation is impaired in Hoxa-10(-/-) mice, and that Hoxa-10 is involved in mediating stromal cell proliferation. Collectively, these results suggest that Hoxa-10 mutation causes specific stromal cell defects that can lead to implantation and decidualization defects apparently without perturbing epithelial cell functions.