Project Summary Mammalian embryos have the potential to develop both male and female reproductive tract organs. However, they typically only differentiate one sex type depending on the presence or absence of fetal gonadal hormones. Two paired epithelial ducts form within the mesonephroi (fetal kidneys) in both genetically male and female embryos. The Wolffian ducts form first and with the adjacent mesenchyme differentiate into male organs, including the vasa deferens, epididymis, and seminal vesicle. The Müllerian ducts then form adjacent to the Wolffian ducts. The Müllerian ducts and their adjacent mesenchyme give rise to female organs, including oviduct, uterus, vagina. In male embryos, the testes secrete testosterone to induce the differentiation of the Wolffian ducts. The testes also secrete anti-Müllerian hormone (AMH) that binds receptors expressed in the mesenchyme adjacent to the Müllerian duct, resulting in elimination of the Müllerian duct epithelium. Alterations in Wolffian and Müllerian duct formation or differentiation to a male or female phenotype lead to Differences of Sex Development (DSD). DSDs are conditions that vary from the typical male and female phenotypes. In female embryos, once the two Müllerian ducts have elongated and connected to the urogenital sinus the posterior regions migrate to the midline and fuse to create the bipartite uterus in the mouse and simplex uterus in human. Alterations in these processes are thought to lead to defects in uterine morphogenesis, associated with infertility, high risk pregnancy, and miscarriage. Much progress has been made in understanding the development of the female reproductive tract organs and the elimination of the Müllerian system in male embryos but there are still many basic questions to be answered. Wnt7a encodes a secreted protein that is expressed in the Müllerian duct epithelium. We have discovered that the Müllerian ducts of Wnt7a-null females do not fuse at the midline, resulting in the form