Regulates the GDP/GTP exchange reaction of the Rho proteins by inhibiting the dissociation of GDP from them, and the subsequent binding of GTP to them. Regulates reorganization of the actin cytoskeleton mediated by Rho family members. Interacts with RHOA. Interacts with RAC1. Interacts with RAC2. Interacts with CDC42. Preferentially expressed in hematopoietic cells. Belongs to the Rho GDI family.