Previously we showed that Hand2 mRNA levels dramatically increase in mouse uterine endometrial stromal cells as they undergo decidualization in vivo. However, very little is still known about the expression and function of this transcription factor in the mouse or human uterusdecidualization. Therefore, the current study was undertaken to provide a more detailed assessment of Hand2gene expression and function in the mouse uterus during the periimplantation period and also in mouse plus human endometrial stromal cells during decidualization in vitro. The results show that Hand2 mRNA and protein levels increase in the mouse uterus during decidualization and this does not depend on the presence of a conceptus. Interestingly Hand2 mRNA and protein are present in endometrial stromal cells adjacent to the luminal epithelium in the uterus prior to the onset of implantation. We find that progesterone is likely a regulator of Hand2expression during uterine sensitization of the mouse uterus. Finally, Hand2 expression increases in mouse and human fibroblast cells as they undergo decidualization in vitro. This expression is significantly increased in response to prostaglandin E2. Notably, reduction of Hand2 expression in these cells using shRNA or siRNA approaches, results in the reduced extent of decidualization as shown by the reduced expression ofa subset of decidualization markers. The results of this study support the hypothesis that Hand2 expression not only plays an important role decidualization but may also play a role in obtaining proper progesterone-dependent uterine sensitization required for implantation to begin.