The PAK (p21-activated kinase) family of serine/threonine kinases plays an important role in multiple cellular processes, including cytoskeletal reorganization, MAPK signaling, apoptotic signaling, etc. Several mechanisms that induce PAK activation have been reported. Binding of Rac/cdc42 to the CRIB (or PBD) domain at the N-terminal region of PAK causes autophosphorylation and conformational change of PAK. More recently identified members PAK4, PAK5 and PAK6 have lower sequence similarity with PAK1-3 in the regulatory N-terminal region.