Vertebrate Hh signaling is linked to the primary cilium. (a) The primary cilium is composed of microtubule doublets extending from the basal body and proteins required for its construction include intraflagellar transport (IFT) proteins and Iguana (Igu). Kinesin-3 (Kif3) transports IFT particles towards the cilium tip, and dynein (dyn) returns IFT cargo to the base. (b) When Hh ligands bind to Patched (Ptc), the Hh-Ptc complex appears to exit the primary cilium, allowing translocation and activation of Smoothened (Smo). Smo signals to the Gli transcription factors, possibly through modulation of Kinesin-7 (Kif-7) and suppressor of Fused (Sufu). Gli proteins exit the cilium and enter the nucleus to promote transcription of Hh target genes after undergoing conversion to an activated state at an undefined point in this process.