Human p21-GTPase-activated protein kinase 1 (PAK1) is a functional homolog of STE20 of Saccharomyces cerevisiae. PAK1 is a downstream effector of the Rho-family small GTPases Rac and Cdc42. A serine/threonine kinase, PAK1 induces the formation of filopodia and lamellipodia in Swiss 3T3 cells. PAK1 inhibits myosin light chain kinase, and appears to play a role in the regulation of apoptosis by phosphorylating the death agonist Bad. Membrane targeting of PAK1 induces neurite outgrowth. PAK1 may also regulate NFkB activation.