SMO is the key signal transducer of the Hh pathway. A, In the absence of Hh ligands, the PTCH receptor at the base of the primary cilium suppresses the function of SMO by preventing its entry into the cilium. Without SMO activation, Gli proteins are processed with Su(Fu), GSK3β, PKA, and CKI into a repressor form (Gli-R), which disable the Hh signaling pathway. B, In the presence of Hh, it binds to PTCH. The binding can be repressed by HIP and supported by Cdo, Gas1 and Boc. Upon binding, the Hh/PTCH complex becomes internalized in endosomes and later degraded. Without inhibition from PTCH, SMO becomes activated and facilitated Gli activation n (GliA), which stimulates Hh target gene expression.