PAK5, a member of the PAK family of protein kinases contains a CDC42/Rac1 interactive binding (CRIB) motif at the N-terminus and a Ste20-like kinase domain at the C-terminus. PAK5 preferentially binds to CDC42 in the presence of GTP and the CRIB motif is essential for this interaction. PAK5 operates downstream of Cdc42 and Rac and antagonizes Rho in the pathway, leading to neurite development. PAK5 is a functional protein kinase, but, unlike PAK-I family kinases (PAK1, 2, and 3), the kinase activity of PAK5 does not seem to require the binding of CDC42. PAK5 is highly expressed in mammalian brain but is not expressed in most other tissues. PAK5 colocalizes and binds to both the actin and MT networks and its subcellular localization is regulated during cell cycle progression.