GTPase-activating protein for p21-rac. In the inactive state, the N terminus protrudes into the active site of the Rho-GAP domain, sterically blocking Rac binding. Phospholipid binding to the Phorbol-ester/DAG-type zinc-finger/C1 domain triggers the cooperative dissociation of these interactions, allowing the N-terminus to move out of the active site and thereby activating the enzyme (By similarity). Found in cerebellum and testis. Expressed specifically in late stage spermatocytes. In the cerebellum, emergence of beta-2 isoform coincides with granule cells maturation and exhibits postnatal developmental increases. Expression is specifically reduced in weaver mutant.