ARL13B and ciliopathy: Since both the mutations of the Arl3-GAP RP2 in RP and the Arl3-GEF Arl13B in JSyndrome lead to ciliary defects and ciliopathies, we conclude that the amount of Arl3·GTP needs to be precisely regulated and that both an increase and a decrease of Arl3·GTP is not tolerated for proper function of the cilium.