The Rho family of proteins mediates cytoskeleton organization and cell motility responses. This family contains twenty different members divided into five groups, the Rho-like, Rac-like, Cdc42-like, Rnd and RhoBTB subfamilies. Rho proteins have GTPase activity and bind GTP in their active form and GDP in their inactive form. In their active form they interact with downstream proteins. However, the Rnd subfamily of proteins containing the Rnd1, Rnd2 and RhoE(Rnd3), binds GTP but not GDP and has no GTPase activity. While Rho proteins are required for stress fibre formation in cultured fibroblasts, epithelial and endothelial cells, overexpression of Rnd3 or Rnd1 in the same cells may cause an opposite effect (i.e. decrease in stress fibre). Thus, the Rnd3 protein may act as an antagonist for the Rho proteins either by directly binding to Rho or to other downstream proteins.