Activates CDC42, a member of the Ras-like family of Rho- and Rac proteins, by exchanging bound GDP for free GTP. Plays a role in regulating the actin cytoskeleton and cell shape. Interacts with DBNL/ABP1 and CTTN. Binds CDC42 (By similarity). May interact with CCPG1. Associated with membrane ruffles and lamellipodia. The DH domain is involved in interaction with CCPG1.