Small rho-like GTPase; essential for establishment and maintenance of cell polarity; mediates a late stage of cell fusion during conjugation, focusing and anchoring Fus2p at the zone of cell fusion to facilitate the concentrated release of cell wall remodeling enzymes; acts with ESCRT proteins at sites of nuclear envelope and ER fission; mutants are defective in the organization of both actin and septins; human homolog CDC42 can complement yeast cdc42 mutants