In mammals, there are several identified isoforms of p21 Activated Protein Kinases or PAKs: PAK1 and PAK3 are mostly brain specific, while PAK2 is expressed ubiquitously. Mutations of the gene coding for PAK3 are associated with X linked mental retardation and PAK3 is a key regulator of synapse formation and plasticity in the hippocampus. PAK3 is thought to play a key role in regulation of cell shape and motility as well as cell death. P21-activated kinase (PAK) is actually a family of serine/threonine protein kinases, members of which are activated by small molecular weight GTPases. The three most common isoforms are PAK 1, PAK 2, and PAK 3 (also known as alpha PAK, gamma PAK, and beta PAK, respectively). These kinases contain numerous regulatory elements that trigger diverse signaling processes such as those initiated by activated GTPases, interaction with Src homology 3 (SH3) domains, and caspase mediated proteolytic cleavage. Autophosphorylation of serine 141 (serine 144 for PAK 1 and serine 139 PAK 3), catalyzed by Cdc42, is required for activation of PAK.