Tetratricopeptide repeat domain 21bTetratricopeptide repeat domain 21B is a protein that in humans is encoded by the TTC21B gene.FunctionThis gene encodes a member of TTC21 family, containing several tetratricopeptide repeat domains. This protein is localized to the cilium axoneme, and may play a role in retrograde intraflagellar transport in cilia. Mutations in this gene are associated with various ciliopathies, nephronophthisis 12, and asphyxiating thoracic dystrophy 4. .