AbstractDefinitive hematopoietic stem and progenitor cells (HSCs/Ps) originating from the yolk sac and/or para-aorta-splanchno-pleura/aorta-gonad-mesonephros are hypothesized to colonize the fetal liver, but mechanisms involved are poorly defined. The Rac subfamily of Rho GTPases has been shown to play essential roles in HSC/P localization to the bone marrow following transplantation. Here, we study the role of Rac1 in HSC/P migration during ontogeny and seeding of fetal liver. Using a triple-transgenic approach, we have deleted Rac1 in HSCs/Ps during very early embryonic development. Without Rac1, there was a decrease in circulating HSCs/Ps in the blood of embryonic day (E) 10.5 embryos, while yolk sac definitive hematopoiesis was quantitatively normal. Intraembryonic hematopoiesis was significantly impaired in Rac1-deficient embryos, culminating with absence of intra-aortic clusters and fetal liver hematopoiesis. At E10.5, Rac1-deficient HSCs/Ps displayed decreased transwell migration and impaired inter-action with the microenvironment in migration-dependent assays. These data suggest that Rac1 plays an important role in HSC/P migration during embryonic development and is essential for the emergence of intraembryonic hematopoiesis.