Fig. 1.

Hh signalling—a simplified model. In the absence of Hh ligand (Hh), the pathway is inactive. Ptch1 inhibits the activity of Smo, thus the pathway activating Gli transcription factors is prevented from entering the nucleus and the Hh target genes are repressed. Activation of the pathway is initiated upon Hh binding to Ptch1, which leads to de-repression of Smo. As a consequence, a signalling cascade involving a multi-protein complex and the primary cilium as the processing platform leads to the translocation of the active form of Gli transcriptional activators to the nucleus. The Hh/Gli target genes are activated including Ptch1 and Gli1 itself. IFT, intraflagellar proteins; PKA, protein kinase A; Sufu, suppressor of fused.