Acts as a negative regulator of GTPase CDC42 by sequestering CDC42-guanine exchange factor DOCK8. Probably by preventing CDC42 activation, negatively regulates CD4(+) T-cell migration in response to chemokine stimulation. Interacts (via LRR repeats) with unphosphorylated DOCK8 (via DHR-2 domain); the interaction prevents the association between DOCK8 and CDC42. No visible phenotype. Mice are viable, fertile and have normal CD4(+) T-cell populations in lymph nodes and spleen. In an experimental autoimmune encephalomyelitis (EAE) disease model, the symptoms, such as paralysis, are more severe. Extended N-terminus.