TTC21B and Joubert syndrome: The tetratricopeptide repeat domain 21B (TTC21B) gene encodes the ciliary protein intraflagellar transport protein 139 (IFT139) and has been recently implicated in heterogeneous diseases, including nephronophthisis type 12 (NPHP12), short-rib thoracic dysplasia 4 (SRTD4), and Joubert syndrome (JBTS).1