The Rnd proteins, which form a distinct sub-group of the Rho family of small GTP-binding proteins, have been shown to regulate the organization of the actin cytoskeleton in several tissues. In the brain, they participate in neurite extension, whereas in smooth muscle, they modulate contractility. Recent evidence has shown that Rnd3 (RhoE) is also involved in the regulation of cell-cycle progression and transformation, indicating that these proteins might have other, as yet unexplored roles.