Serine/threonine-protein kinases PAK1, 2 and 3 (PAK1,2,3) serve as downstream effectors of RACs in regulating spine and synapse formation. Once activated, Ras-related C3 botulinum toxin substrate 1 (RAC1) binds to a variety of downstream effector proteins. Among the most well characterized effectors are PAKs. In addition to binding active RAC, PAKs also directly bind p-21-activated kinase (PAK) interacting exchange factor (beta-PIX, bPIX aka ARHGEF7) (Zhang et al. 2005).