Endoglin (CD105) is expressed on the surface of endothelial and haematopoietic cells in mammals and binds TGFβ isoforms 1 and 3 in combination with the signaling complex of TGFβ receptors types I and II. Endoglin expression increases during angiogenesis, wound healing, and inflammation, all of which are associated with TGFβ signaling and alterations in vascular structure. The importance of endoglin for normal vascular architecture is further indicated by the association of mutations in the endoglin gene with the inherited disorder Hereditary Haemorrhagic Telangiectasia Type 1 (HHT1), a disease characterised by bleeding from vascular malformations. In order to study the role of endoglin in vivo in more detail and to work toward developing an animal model of HHT1, we have derived mice that carry a targeted nonsense mutation in the endoglin gene. Studies on these mice have revealed that endoglin is essential for early development. Embryos homozygous for the endoglin mutation fail to progress beyond 10.5 days postcoitum and fail to form mature blood vessels in the yolk sac. This phenotype is remarkably similar to that of the TGFβ1 and the TGFβ receptor II knockout mice, indicating that endoglin is needed in vivo for TGFβ1 signaling during extraembryonic vascular development. In addition, we have observed cardiac defects in homozygous endoglin-deficient embryos, suggesting endoglin also plays a role in cardiogenesis. We anticipate that heterozygous mice will ultimately serve as a useful disease model for HHT1, as some individuals have dilated and fragile blood vessels similar to vascular malformations seen in HHT patients.