PROS1 and Joubert syndrome: The perspective that dysregulated axoneme glutamylation might be a central etiology in JBTS was further strengthened by the finding that the Joubert syndrome protein ARL13B associates with FIP5, a known effector of another ciliary GTPase RAB11, to promote the ciliary import of tubulin glutamylases TTLL5 and TTLL6 in human epithelial cells (He et al., 2018).