ADP-ribosylation factor-like protein 13B (ARL13B), also known as ADP-ribosylation factor-like protein 2-like 1, is a protein that in humans is encoded by the ARL13B gene. This gene encodes a member of the ADP-ribosylation factor-like family. The encoded protein is a small GTPase that contains both N-terminal and C-terminal guanine nucleotide-binding motifs. This protein is localized in the cilia and plays a role in cilia formation and in maintenance of cilia. Mutations in this gene are the cause of Joubert syndrome 8. Alternate splicing results in multiple transcript variants.