The p21-activated kinases (PAKs) are serine-threonine kinases that bind to the active forms of Cdc42 and Rac.  They are divided into two groups, the first of which include PAK1, 2 and 3, and can be activated by Cdc42/Rac binding.  Group 1 PAKs contain an autoinhibitory domain whose activity is regulated by Cdc42/Rac binding.  The group 1 PAKs are known to be involved in cellular processes such as gene transcription, apoptosis, and cell morphology and motility.  Much less is known about the second group, which includes PAK4, 5 and 6.  These proteins are not activated by Cdc42/Rac binding.  PAK6 was initially identified as an androgen receptor in a yeast two hybrid screen and was found to be highly expressed in testis and prostate tissues.  Later experiments have shown it to be activated by MAP kinase kinase 6 and p38 MAP kinase, suggesting that PAK6 may play a role in the cellular response to stress-related signals.