Heterotrimeric GTP-binding proteins (G proteins) participate in cellular signaling and regulate a variety of physiological processes. Disruption of the gene encoding the G protein subunit α₁₃ (Gα₁₃) in mice impaired the ability of endothelial cells to develop into an organized vascular system, resulting in intrauterine death. In addition, Gα₁₃ (−/−) embryonic fibroblasts showed greatly impaired migratory responses to thrombin. These results demonstrate that Gα₁₃ participates in the regulation of cell movement in response to specific ligands, as well as in developmental angiogenesis.