Fmr1 knockout mice cause an overactivation of Rac1 (or increased levels of membrane-bound Rac1) and its effector PAK1 and cofilin phosphorylation in the brain, with FXS phenotypes including hyperactivity, learning deficits and macroorchidism [224,225,226,227,228]. Here, PAK1 is linked to fragile X syndrome.