Serine/threonine-protein kinase PAK 1 is an enzyme that in humans is encoded by the PAK1 gene. PAK1 is one of six members of the PAK family of serine/threonine kinases which are broadly divided into group I (PAK1, PAK2 and PAK3) and group II (PAK4, PAK6 and PAK5/7). The PAKs are evolutionarily conserved. PAK1 localizes in distinct sub-cellular domains in the cytoplasm and nucleus. PAK1 regulates cytoskeleton remodeling, phenotypic signaling and gene expression, and affects a wide variety of cellular processes such as directional motility, invasion, metastasis, growth, cell cycle progression, angiogenesis. PAK1-signaling dependent cellular functions regulate both physiologic and disease processes, including cancer, as PAK1 is widely overexpressed and hyperstimulated in human cancer, at-large. PAK2 and its cleaved fragment localize in both the cytoplasmic or nuclear compartments. PAK2 signaling modulates apoptosis, endothelial lumen formation, viral pathogenesis, and cancer including, breast, hepatocarcinoma, and gastric and cancer, at-large. PAK3 is preferentially expressed in neuronal cells and involved in synapse formation and plasticity and mental retardation.