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. Binding of Rac/cdc42 to the CRIB (or PBD) domain at the N-terminal region of PAK causes autophosphorylation and conformational change of PAK. Phosphorylation of Ser21 of PAK1 or Ser20 of PAK2 regulates its binding with the adaptor protein Nck.