The GTPases constitute a very large protein family, whose members are involved in the control of cell growth, transport of molecules, synthesis of other proteins, etc. Despite the many functions of the GTPases, they follow a common cyclic pattern (Figure 1). The activity of the GTPases is regulated by factors that control their ability to bind and hydrolyse guanosine triphosphate (GTP) to guanosine diphosphate (GDP). So far, it has been the general assumption that a GTPase is active or “on” when it is bound to GTP and inactive or “off” in complex with GDP. The GTPases are therefore sometimes referred to as molecular “switches.”