GTPase-activating protein for p21-rac and a phorbol ester receptor. May play an important role in neuronal signal-transduction mechanisms (By similarity). Involved in the assembly of neuronal locomotor circuits as a direct effector of EPHA4 in axon guidance. Interacts with EPHA4; effector of EPHA4 in axon guidance linking EPHA4 activation to RAC1 regulation. May also interact with EPHB1 and EPHB2. Phosphorylated. Phosphorylation is EPHA4 kinase activity-dependent. Mice are viable and fertile but display a loss of coordination of limb movement which phenocopies the one of Epha4 mutant mice.