Plasma membrane-associated small GTPase which cycles between an active GTP-bound and an inactive GDP-bound state. In active state binds to a variety of effector proteins to regulate cellular responses. Involved in epithelial cell polarization processes. May play a role in CFTR trafficking to the plasma membrane. Causes the formation of thin, actin-rich surface projections called filopodia (By similarity). Regulated by guanine nucleotide exchange factors (GEFs) which promote the exchange of bound GDP for free GTP, GTPase activating proteins (GAPs) which increase the GTP hydrolysis activity, and GDP dissociation inhibitors which inhibit the dissociation of the nucleotide from the GTPase. Interacts with EXO70, CDC42EP1, CDC42EP2 and CDC42EP3 in a GTP-dependent manner (By similarity). Interacts with CDC42EP4, PARD6A, PARD6G (and probably PARD6B) in a GTP-dependent manner. Part of a quaternary complex containing PARD3, some PARD6 protein (PARD6A, PARD6B or PARD6G) and some atypical PKC protein (PRKCI or PRKCZ). Interacts with GOPC (By similarity). Interacts with ARHGAP33/TCGAP. May be post-translationally modified by both palmitoylation and polyisoprenylation. Belongs to the small GTPase superfamily. Rho family.