Rho family small GTPases, including Rho, Rac and cdc42, act as molecular switches, regulating processes such as cell migration, adhesion, proliferation and differentiation. They are activated by guanine nucleotide exchange factors (GEFs), which catalyze the exchange of bound GDP for GTP, and inhibited by GTPase activating proteins (GAPs), which catalyze the hydrolysis of GTP to GDP. A third level of regulation is provided by the stoichiometric binding of Rho GDP dissociation inhibitor (RhoGDI). RhoGDI affects Rho activity by inhibiting nucleotide exchange and membrane association, regulating activity and localization. The inhibitory and shuttling functions of RhoGDI have been uncoupled using mutant forms of RhoGDI. Phosphorylation of GDIs and/or GTPases can modulate their affinity for each other and, therefore, GTPase mediated signaling. PAK1 phosphorylation of RhoGDI at serines 101 and 174 causes release and activation of Rac1, but not RhoA.