Fig. 1: Schematic representation of the Hedgehog (Hh) signalling pathway in Drosophila.

Briefly, in Drosophila, the core components of the Hh pathway include Hh ligands; the Patched receptor (Ptc); the transcription factor cubitus interruptus (Ci); the signalling activator G-protein-coupled receptor-like protein Smoothened (Smo); and a signal transducer, such as Costal (Cos2), Fused (Fu) or Suppressor of Fused (SuFu). a Off-state. The Hh signalling pathway is inactive in the absence of a Hh ligand. The seven-pass transmembrane protein Smo is repressed by Ptc. The target genes of the Hh signalling pathway are not transcribed or expressed. b On-state. In the presence of the Hh ligand, the Hh ligand binds to Ptc, and the repression of Smo is alleviated, initiating the classical Hh signalling pathway. The cascade of cytoplasmic protein complexes (such as Su, SuFu and Cos2) is activated, and the downstream transcription factor Ci translocates to the nucleus, leading to the transcription of target genes. Hh Hedgehog ligand, Smo Smoothened, PTCH1 Patched1, GLI glioma-associated oncogene family zinc finger