p21-activated kinases (PAKs) belong to the family of serine/threonine kinases involved in the control of various cellular processes, including the cell cycle, dynamics of the cytoskeleton, apoptosis, oncogenic transformation, and transcription. All PAK family members are characterized by the presence of p21-binding domain. p21-activated kinases are regulated by the small GTP-binding proteins Rac and Cdc42, and lipids, which stimulate autophosphorylation and phosphorylation of exogenous substrates. Serine (Ser-474) is the likely autophosphorylation site in the kinase domain of PAK4 in vivo. Phosphospecific antibodies directed against serine 474 detect activated PAK4 on the Golgi membrane when PAK4 is co-expressed with activated Cdc42. Current data strongly implicates PAK-4 in oncogenesis. PAK4 is frequently overexpressed in human tumor cell lines of various tissue origins.